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>

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