fix(server): add middleware for provider data and test context (#4367)

# What does this PR do?
Consolidates provider data context handling into middleware, eliminating
duplication between FastAPI router routes and legacy @webmethod routes.

Closes #4366 

## Test Plan

Added unit test suite `test_test_context_middleware`, specifically
`test_middleware_extracts_test_id_from_header` to validate the expected
behavior.
```
❯ ./scripts/unit-tests.sh tests/unit/
```

Integration of the middleware test context with the `files` FastAPI
router migration from
[pull/4339](https://github.com/llamastack/llama-stack/pull/4339).
```
❯ git switch migrate-files-api
Switched to branch 'migrate-files-api'
❯ git rebase fix-test-ctx-middleware
Successfully rebased and updated refs/heads/migrate-files-api.
❯ ./scripts/integration-tests.sh --inference-mode replay --suite base --setup ollama --stack-config server:starter --subdirs files
```

Signed-off-by: Matthew F Leader <mleader@redhat.com>

src/llama_stack/core/server/server.py

@@ -235,35 +235,18 @@ async def log_request_pre_validation(request: Request):
 def create_dynamic_typed_route(func: Any, method: str, route: str) -> Callable:
     @functools.wraps(func)
     async def route_handler(request: Request, **kwargs):
-        # Get auth attributes from the request scope
-        user = user_from_scope(request.scope)
-
         await log_request_pre_validation(request)
 
-        test_context_token = None
-        test_context_var = None
-        reset_test_context_fn = None
-
-        # Use context manager with both provider data and auth attributes
-        with request_provider_data_context(request.headers, user):
-            if os.environ.get("LLAMA_STACK_TEST_INFERENCE_MODE"):
-                from llama_stack.core.testing_context import (
-                    TEST_CONTEXT,
-                    reset_test_context,
-                    sync_test_context_from_provider_data,
-                )
-
-                test_context_token = sync_test_context_from_provider_data()
-                test_context_var = TEST_CONTEXT
-                reset_test_context_fn = reset_test_context
-
         is_streaming = is_streaming_request(func.__name__, request, **kwargs)
 
         try:
             if is_streaming:
+                # Preserve context vars across async generator boundaries
                 context_vars = [PROVIDER_DATA_VAR]
-                    if test_context_var is not None:
+                if os.environ.get("LLAMA_STACK_TEST_INFERENCE_MODE"):
-                        context_vars.append(test_context_var)
+                    from llama_stack.core.testing_context import TEST_CONTEXT
+
+                    context_vars.append(TEST_CONTEXT)
                 gen = preserve_contexts_async_generator(sse_generator(func(**kwargs)), context_vars)
                 return StreamingResponse(gen, media_type="text/event-stream")
             else:
@@ -282,9 +265,6 @@ def create_dynamic_typed_route(func: Any, method: str, route: str) -> Callable:
             else:
                 logger.error(f"Error executing endpoint {route=} {method=}: {str(e)}")
             raise translate_exception(e) from e
-            finally:
-                if test_context_token is not None and reset_test_context_fn is not None:
-                    reset_test_context_fn(test_context_token)
 
     sig = inspect.signature(func)
 
@@ -356,6 +336,42 @@ class ClientVersionMiddleware:
         return await self.app(scope, receive, send)
 
 
+class ProviderDataMiddleware:
+    """Middleware to set up request context for all routes.
+
+    Sets up provider data context from X-LlamaStack-Provider-Data header
+    and auth attributes. Also handles test context propagation when
+    running in test mode for deterministic ID generation.
+    """
+
+    def __init__(self, app):
+        self.app = app
+
+    async def __call__(self, scope, receive, send):
+        if scope["type"] == "http":
+            headers = {k.decode(): v.decode() for k, v in scope.get("headers", [])}
+            user = user_from_scope(scope)
+
+            with request_provider_data_context(headers, user):
+                test_context_token = None
+                reset_fn = None
+                if os.environ.get("LLAMA_STACK_TEST_INFERENCE_MODE"):
+                    from llama_stack.core.testing_context import (
+                        reset_test_context,
+                        sync_test_context_from_provider_data,
+                    )
+
+                    test_context_token = sync_test_context_from_provider_data()
+                    reset_fn = reset_test_context
+                try:
+                    return await self.app(scope, receive, send)
+                finally:
+                    if test_context_token and reset_fn:
+                        reset_fn(test_context_token)
+
+        return await self.app(scope, receive, send)
+
+
 def create_app() -> StackApp:
     """Create and configure the FastAPI application.
 
@@ -395,6 +411,8 @@ def create_app() -> StackApp:
     if not os.environ.get("LLAMA_STACK_DISABLE_VERSION_CHECK"):
         app.add_middleware(ClientVersionMiddleware)
 
+    app.add_middleware(ProviderDataMiddleware)
+
     impls = app.stack.impls
 
     if config.server.auth:

tests/unit/server/test_test_context_middleware.py

@@ -0,0 +1,106 @@
+# 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.
+
+import json
+import os
+
+import pytest
+from fastapi import APIRouter, FastAPI
+from starlette.testclient import TestClient
+
+from llama_stack.core.server.server import ProviderDataMiddleware
+from llama_stack.core.testing_context import get_test_context
+
+
+@pytest.fixture
+def app_with_middleware():
+    """Create a minimal FastAPI app with ProviderDataMiddleware."""
+    app = FastAPI()
+
+    router = APIRouter()
+
+    @router.get("/test-context")
+    def get_current_test_context():
+        return {"test_id": get_test_context()}
+
+    app.include_router(router)
+    app.add_middleware(ProviderDataMiddleware)
+
+    return app
+
+
+@pytest.fixture
+def test_mode_env(monkeypatch):
+    """Set environment variables required for test context extraction."""
+    monkeypatch.setenv("LLAMA_STACK_TEST_INFERENCE_MODE", "replay")
+    monkeypatch.setenv("LLAMA_STACK_TEST_STACK_CONFIG_TYPE", "server")
+
+
+def test_middleware_returns_none_without_header(app_with_middleware, test_mode_env):
+    """Without the provider data header, test context should be None."""
+    client = TestClient(app_with_middleware)
+    response = client.get("/test-context")
+
+    assert response.status_code == 200
+    assert response.json()["test_id"] is None
+
+
+def test_middleware_extracts_test_id_from_header(app_with_middleware, test_mode_env):
+    """With the provider data header containing __test_id, it should be extracted."""
+    client = TestClient(app_with_middleware)
+
+    provider_data = json.dumps({"__test_id": "test-abc-123"})
+    response = client.get(
+        "/test-context",
+        headers={"X-LlamaStack-Provider-Data": provider_data},
+    )
+
+    assert response.status_code == 200
+    assert response.json()["test_id"] == "test-abc-123"
+
+
+def test_middleware_handles_empty_provider_data(app_with_middleware, test_mode_env):
+    """Empty provider data should result in None test context."""
+    client = TestClient(app_with_middleware)
+
+    response = client.get(
+        "/test-context",
+        headers={"X-LlamaStack-Provider-Data": "{}"},
+    )
+
+    assert response.status_code == 200
+    assert response.json()["test_id"] is None
+
+
+def test_middleware_handles_invalid_json(app_with_middleware, test_mode_env):
+    """Invalid JSON in header should not crash, test context should be None."""
+    client = TestClient(app_with_middleware)
+
+    response = client.get(
+        "/test-context",
+        headers={"X-LlamaStack-Provider-Data": "not-valid-json"},
+    )
+
+    assert response.status_code == 200
+    assert response.json()["test_id"] is None
+
+
+def test_middleware_noop_without_test_mode(app_with_middleware):
+    """Without test mode env vars, middleware should not extract test context."""
+    # Ensure env vars are not set
+    os.environ.pop("LLAMA_STACK_TEST_INFERENCE_MODE", None)
+    os.environ.pop("LLAMA_STACK_TEST_STACK_CONFIG_TYPE", None)
+
+    client = TestClient(app_with_middleware)
+
+    provider_data = json.dumps({"__test_id": "test-abc-123"})
+    response = client.get(
+        "/test-context",
+        headers={"X-LlamaStack-Provider-Data": provider_data},
+    )
+
+    assert response.status_code == 200
+    assert response.json()["test_id"] is None