feat(gateway): X-API-Deprecated header for legacy /api/* paths (Stage 1 PR5)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_api_deprecation_header.py

@@ -0,0 +1,46 @@
+"""Deprecation header tests (Stage 1 PR5)."""
+
+from __future__ import annotations
+
+from starlette.testclient import TestClient
+
+
+def _make_app():
+    from fastapi import FastAPI
+
+    from app.gateway.deprecation_middleware import ApiDeprecationMiddleware
+
+    app = FastAPI()
+    app.add_middleware(ApiDeprecationMiddleware)
+
+    @app.get("/api/threads")
+    async def legacy():
+        return {"ok": True}
+
+    @app.get("/api/v1/threads")
+    async def versioned():
+        return {"ok": True}
+
+    @app.get("/api/langgraph/info")
+    async def lg():
+        return {"ok": True}
+
+    return app
+
+
+def test_legacy_path_gets_deprecation_header():
+    client = TestClient(_make_app())
+    r = client.get("/api/threads")
+    assert r.headers.get("X-API-Deprecated") == "2027-01-01"
+
+
+def test_versioned_path_no_header():
+    client = TestClient(_make_app())
+    r = client.get("/api/v1/threads")
+    assert "X-API-Deprecated" not in r.headers
+
+
+def test_langgraph_path_no_header():
+    client = TestClient(_make_app())
+    r = client.get("/api/langgraph/info")
+    assert "X-API-Deprecated" not in r.headers