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/models.py

@@ -47,6 +47,6 @@ class EntityRelationship(Base, TenantMixin, OwnedMixin):
     relationship_type: Mapped[str] = mapped_column(
         String(50), nullable=False, comment="Type of relationship (e.g. 'works_for', 'has_email', 'related_to')"
     )
-    metadata: Mapped[dict[str, Any] | None] = mapped_column(
-        JSONB, nullable=True, default=dict, comment="Arbitrary metadata about the relationship"
+    meta: Mapped[dict[str, Any] | None] = mapped_column(
+        "metadata", JSONB, nullable=True, default=dict, comment="Arbitrary metadata about the relationship"
     )