feat: Implement FastAPI router system (#4191)

# What does this PR do?

This commit introduces a new FastAPI router-based system for defining
API endpoints, enabling a migration path away from the legacy @webmethod
decorator system. The implementation includes router infrastructure,
migration of the Batches API as the first example, and updates to
server, OpenAPI generation, and inspection systems to support both
routing approaches.

The router infrastructure consists of a router registry system that
allows APIs to register FastAPI router factories, which are then
automatically discovered and included in the server application.
Standard error responses are centralized in router_utils to ensure
consistent OpenAPI specification generation with proper $ref references
to component responses.

The Batches API has been migrated to demonstrate the new pattern. The
protocol definition and models remain in llama_stack_api/batches,
maintaining clear separation between API contracts and server
implementation. The FastAPI router implementation lives in
llama_stack/core/server/routers/batches, following the established
pattern where API contracts are defined in llama_stack_api and server
routing logic lives in
llama_stack/core/server.

The server now checks for registered routers before falling back to the
legacy webmethod-based route discovery, ensuring backward compatibility
during the migration period. The OpenAPI generator has been updated to
handle both router-based and webmethod-based routes, correctly
extracting metadata from FastAPI route decorators and Pydantic Field
descriptions. The inspect endpoint now includes routes from both
systems, with proper filtering for deprecated routes and API levels.

Response descriptions are now explicitly defined in router decorators,
ensuring the generated OpenAPI specification matches the previous
format. Error responses use $ref references to component responses
(BadRequest400, TooManyRequests429, etc.) as required by the
specification. This is neat and will allow us to remove a lot of boiler
plate code from our generator once the migration is done.

This implementation provides a foundation for incrementally migrating
other APIs to the router system while maintaining full backward
compatibility with existing webmethod-based APIs.

Closes: https://github.com/llamastack/llama-stack/issues/4188

## Test Plan

CI, the server should start, same routes should be visible.

```
curl http://localhost:8321/v1/inspect/routes | jq '.data[] | select(.route | contains("batches"))'
```

Also:

```
 uv run pytest tests/integration/batches/ -vv --stack-config=http://localhost:8321
================================================== test session starts ==================================================
platform darwin -- Python 3.12.8, pytest-8.4.2, pluggy-1.6.0 -- /Users/leseb/Documents/AI/llama-stack/.venv/bin/python3
cachedir: .pytest_cache
metadata: {'Python': '3.12.8', 'Platform': 'macOS-26.0.1-arm64-arm-64bit', 'Packages': {'pytest': '8.4.2', 'pluggy': '1.6.0'}, 'Plugins': {'anyio': '4.9.0', 'html': '4.1.1', 'socket': '0.7.0', 'asyncio': '1.1.0', 'json-report': '1.5.0', 'timeout': '2.4.0', 'metadata': '3.1.1', 'cov': '6.2.1', 'nbval': '0.11.0'}}
rootdir: /Users/leseb/Documents/AI/llama-stack
configfile: pyproject.toml
plugins: anyio-4.9.0, html-4.1.1, socket-0.7.0, asyncio-1.1.0, json-report-1.5.0, timeout-2.4.0, metadata-3.1.1, cov-6.2.1, nbval-0.11.0
asyncio: mode=Mode.AUTO, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 24 items                                                                                                      

tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_creation_and_retrieval[None] SKIPPED [  4%]
tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_listing[None] SKIPPED               [  8%]
tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_immediate_cancellation[None] SKIPPED [ 12%]
tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_e2e_chat_completions[None] SKIPPED  [ 16%]
tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_e2e_completions[None] SKIPPED       [ 20%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_invalid_endpoint[None] SKIPPED [ 25%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_cancel_completed[None] SKIPPED [ 29%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_missing_required_fields[None] SKIPPED [ 33%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_invalid_completion_window[None] SKIPPED [ 37%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_streaming_not_supported[None] SKIPPED [ 41%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_mixed_streaming_requests[None] SKIPPED [ 45%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_endpoint_mismatch[None] SKIPPED [ 50%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_missing_required_body_fields[None] SKIPPED [ 54%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_invalid_metadata_types[None] SKIPPED [ 58%]
tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_e2e_embeddings[None] SKIPPED        [ 62%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_nonexistent_file_id PASSED [ 66%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_malformed_jsonl PASSED     [ 70%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_file_malformed_batch_file[empty] XFAIL [ 75%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_file_malformed_batch_file[malformed] XFAIL [ 79%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_retrieve_nonexistent PASSED [ 83%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_cancel_nonexistent PASSED  [ 87%]
tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_error_handling_invalid_model PASSED [ 91%]
tests/integration/batches/test_batches_idempotency.py::TestBatchesIdempotencyIntegration::test_idempotent_batch_creation_successful PASSED [ 95%]
tests/integration/batches/test_batches_idempotency.py::TestBatchesIdempotencyIntegration::test_idempotency_conflict_with_different_params PASSED [100%]

================================================= slowest 10 durations ==================================================
1.01s call     tests/integration/batches/test_batches_idempotency.py::TestBatchesIdempotencyIntegration::test_idempotent_batch_creation_successful
0.21s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_nonexistent_file_id
0.17s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_malformed_jsonl
0.12s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_error_handling_invalid_model
0.05s setup    tests/integration/batches/test_batches.py::TestBatchesIntegration::test_batch_creation_and_retrieval[None]
0.02s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_file_malformed_batch_file[empty]
0.01s call     tests/integration/batches/test_batches_idempotency.py::TestBatchesIdempotencyIntegration::test_idempotency_conflict_with_different_params
0.01s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_file_malformed_batch_file[malformed]
0.01s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_retrieve_nonexistent
0.00s call     tests/integration/batches/test_batches_errors.py::TestBatchesErrorHandling::test_batch_cancel_nonexistent
======================================= 7 passed, 15 skipped, 2 xfailed in 1.78s ========================================
```

---------

Signed-off-by: Sébastien Han <seb@redhat.com>

src/llama_stack_api/batches/__init__.py

@@ -0,0 +1,40 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""Batches API protocol and models.
+
+This module contains the Batches protocol definition.
+Pydantic models are defined in llama_stack_api.batches.models.
+The FastAPI router is defined in llama_stack_api.batches.fastapi_routes.
+"""
+
+from openai.types import Batch as BatchObject
+
+# Import fastapi_routes for router factory access
+from . import fastapi_routes
+
+# Import protocol for re-export
+from .api import Batches
+
+# Import models for re-export
+from .models import (
+    CancelBatchRequest,
+    CreateBatchRequest,
+    ListBatchesRequest,
+    ListBatchesResponse,
+    RetrieveBatchRequest,
+)
+
+__all__ = [
+    "Batches",
+    "BatchObject",
+    "CancelBatchRequest",
+    "CreateBatchRequest",
+    "ListBatchesRequest",
+    "ListBatchesResponse",
+    "RetrieveBatchRequest",
+    "fastapi_routes",
+]