test(gateway): strengthen dual-mount coverage to a v1-twin invariant (Stage 1 PR5)

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

backend/tests/test_api_deprecation_header.py

@@ -25,6 +25,10 @@ def _make_app():
     async def lg():
         return {"ok": True}
 
+    @app.get("/api/assistants/info")
+    async def assistants():
+        return {"ok": True}
+
     return app
 
 
@@ -44,3 +48,12 @@ def test_langgraph_path_no_header():
     client = TestClient(_make_app())
     r = client.get("/api/langgraph/info")
     assert "X-API-Deprecated" not in r.headers
+
+
+def test_assistants_compat_path_gets_deprecation_header():
+    # assistants_compat is an un-versioned LangGraph-platform stub; it
+    # intentionally carries the deprecation header (it is /api/, not
+    # /api/v1 or /api/langgraph). Documented here to prevent confusion.
+    client = TestClient(_make_app())
+    r = client.get("/api/assistants/info")
+    assert r.headers.get("X-API-Deprecated") == "2027-01-01"

backend/tests/test_api_v1_dual_mount.py

@@ -25,11 +25,16 @@ def test_runs_dual_mounted():
     assert "/api/v1/runs/stream" in paths
 
 
-def test_threads_dual_mounted():
+def test_every_legacy_api_path_has_v1_twin():
+    """Strong invariant: every unversioned /api/* path (except the
+    intentionally-excluded surfaces) must also exist under /api/v1/*.
+    Catches any single legacy router losing its v1 mount."""
     paths = _paths()
-    # at least one threads sub-path must exist on both surfaces
-    assert any(p.startswith("/api/threads/") for p in paths)
-    assert any(p.startswith("/api/v1/threads/") for p in paths)
+    excluded_prefixes = ("/api/v1/", "/api/langgraph/", "/api/assistants")
+    legacy = {p for p in paths if p.startswith("/api/") and not p.startswith(excluded_prefixes)}
+    assert legacy, "expected some unversioned /api/* paths"
+    missing = sorted(p for p in legacy if ("/api/v1/" + p[len("/api/") :]) not in paths)
+    assert missing == [], f"legacy /api paths without an /api/v1 twin: {missing}"
 
 
 def test_uploads_dual_mounted():