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/graph_rag/routes.py

@@ -108,7 +108,7 @@ async def list_relationships(
                 "target_type": r.target_type,
                 "target_id": str(r.target_id),
                 "relationship_type": r.relationship_type,
-                "metadata": r.metadata,
+                "metadata": r.meta,
                 "owner_id": str(r.owner_id) if r.owner_id else None,
                 "created_at": r.created_at.isoformat() if r.created_at else None,
             }