test: Add 126 tests for Phase 5 plugins + fix 2 source bugs

Tests (5 files, 126 tests, all passing):
- test_agent_memory.py: 22 tests (store, retrieve, delete, routes, tenant isolation)
- test_graph_rag.py: 22 tests (create, traverse BFS, bidirectional, max_hops, cycles, routes)
- test_marketplace.py: 26 tests (fetch, download, verify, install, categories, routes)
- test_agent_subtasks.py: 25 tests (create, wait, cancel, aggregate, list, model)
- test_external_agent_api.py: 31 tests (run, status, stream, auth, rate limit)

Bugfixes:
- graph_rag/models.py: metadata -> meta (SQLAlchemy reserved attribute)
- marketplace/routes.py: fix default parameter validation

app/plugins/builtins/marketplace/routes.py

@@ -96,7 +96,7 @@ async def get_marketplace_listing(
 @router.post("/install/{name}", response_model=MarketplaceInstallResponse)
 async def install_from_marketplace(
     name: str,
-    body: MarketplaceInstallRequest = MarketplaceInstallRequest(name=""),
+    body: MarketplaceInstallRequest = MarketplaceInstallRequest.model_construct(name=""),
     db: AsyncSession = Depends(get_db),
     current_user: dict = Depends(require_admin),
 ):
@@ -138,7 +138,7 @@ async def install_from_marketplace(
 @router.post("/verify/{name}", response_model=MarketplaceVerifyResponse)
 async def verify_plugin_signature(
     name: str,
-    body: MarketplaceInstallRequest = MarketplaceInstallRequest(name=""),
+    body: MarketplaceInstallRequest = MarketplaceInstallRequest.model_construct(name=""),
     db: AsyncSession = Depends(get_db),
     current_user: dict = Depends(require_permission("marketplace:read")),
 ):