Strip /api prefix from 13 legacy router APIRouter() declarations and dual-mount
each on prefix="/api" (backward compat) and prefix="/api/v1" (versioned surface)
in app.py. Auth, service-accounts, api-keys, assistants-compat remain single-mount.
Update 10 test files to pass prefix="/api" when directly including stripped routers.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
`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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