feat(openapi): switch to fastapi-based generator (#3944)

# What does this PR do?
This replaces the legacy "pyopenapi + strong_typing" pipeline with a
FastAPI-backed generator that has an explicit schema registry inside
`llama_stack_api`. The key changes:

1. **New generator architecture.** FastAPI now builds the OpenAPI schema
directly from the real routes, while helper modules
(`schema_collection`, `endpoints`, `schema_transforms`, etc.)
post-process the result. The old pyopenapi stack and its strong_typing
helpers are removed entirely, so we no longer rely on fragile AST
analysis or top-level import side effects.

2. **Schema registry in `llama_stack_api`.** `schema_utils.py` keeps a
`SchemaInfo` record for every `@json_schema_type`, `register_schema`,
and dynamically created request model. The OpenAPI generator and other
tooling query this registry instead of scanning the package tree,
producing deterministic names (e.g., `{MethodName}Request`), capturing
all optional/nullable fields, and making schema discovery testable. A
new unit test covers the registry behavior.

3. **Regenerated specs + CI alignment.** All docs/Stainless specs are
regenerated from the new pipeline, so optional/nullable fields now match
reality (expect the API Conformance workflow to report breaking
changes—this PR establishes the new baseline). The workflow itself is
back to the stock oasdiff invocation so future regressions surface
normally.

*Conformance will be RED on this PR; we choose to accept the
deviations.*

## Test Plan
- `uv run pytest tests/unit/server/test_schema_registry.py`
- `uv run python -m scripts.openapi_generator.main docs/static`

---------

Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

tests/unit/server/test_schema_registry.py

@@ -0,0 +1,48 @@
+# 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.
+
+from pydantic import BaseModel
+
+from llama_stack_api import Conversation, SamplingStrategy
+from llama_stack_api.schema_utils import (
+    clear_dynamic_schema_types,
+    get_registered_schema_info,
+    iter_dynamic_schema_types,
+    iter_json_schema_types,
+    iter_registered_schema_types,
+    register_dynamic_schema_type,
+)
+
+
+def test_json_schema_registry_contains_known_model() -> None:
+    assert Conversation in iter_json_schema_types()
+
+
+def test_registered_schema_registry_contains_sampling_strategy() -> None:
+    registered_names = {info.name for info in iter_registered_schema_types()}
+    assert "SamplingStrategy" in registered_names
+
+    schema_info = get_registered_schema_info(SamplingStrategy)
+    assert schema_info is not None
+    assert schema_info.name == "SamplingStrategy"
+
+
+def test_dynamic_schema_registration_round_trip() -> None:
+    existing_models = tuple(iter_dynamic_schema_types())
+    clear_dynamic_schema_types()
+    try:
+
+        class TemporaryModel(BaseModel):
+            foo: str
+
+        register_dynamic_schema_type(TemporaryModel)
+        assert TemporaryModel in iter_dynamic_schema_types()
+
+        clear_dynamic_schema_types()
+        assert TemporaryModel not in iter_dynamic_schema_types()
+    finally:
+        for model in existing_models:
+            register_dynamic_schema_type(model)