`create_thread` in `app/gateway/routers/threads.py` already relies on
the AUTO sentinel default for ``ThreadMetaStore.create``, so once T6.1
landed the workspace_id was implicitly carried from the contextvar.
This commit adds the integration coverage: TestClient + stub auth
middleware + ``MemoryThreadMetaStore`` round-trip showing the persisted
row carries the right ``workspace_id`` under two distinct workspaces.
No production code change — the route wiring was correct, but the
guarantee was previously only proven at the unit level.
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