A thinking-only STOP on the daily endpoint flipped `started` via
`ensureStarted`, and the endpoint-failover catch guard gated on
`!started`, so Antigravity auto mode never requested the sandbox
endpoint and recorded a false empty-response failure for silent
Advisor turns.
Guard failover on a new `emittedVisibleContent` flag (set only when a
visible text delta or tool call is pushed) instead of `started`, so
hidden thinking no longer blocks the fallback while genuine partial
output still does.
Fixes#8480
