fix(antigravity): raise the fallback client version to 2.9.1 (#5175)

Cloud Code resolves newer Antigravity models only for clients reporting
at least 2.9.0; below that it answers 404 Requested entity was not found.
The offline fallback still reported 2.9.0's predecessor 2.2.1, so every
request sent before the hub manifest is first fetched — or from a
deployment that cannot reach the manifest at all — asked for models such
as gemini-3.7-flash-high with a version the backend rejects.

The hub manifest the updater already polls currently publishes 2.9.1, so
this only aligns the offline floor with what the online path resolves.
The test now asserts the floor rather than a literal, so a future
downgrade below 2.9.0 fails instead of silently reintroducing the 404.