When Antigravity returns 429, throw immediately instead of waiting for the
server-provided retry delay (which can be 10+ minutes). This lets clawdbot
quickly rotate to another account.
This patch was previously in PR #454 but was accidentally removed in 0dbb569
when bumping to pi-ai 0.40.0. The upstream release did NOT include this fix.
Context: Antigravity rate limits cause pi-ai to sleep for the full retry
delay inside the request, blocking the thread. Clawdbot's timeout would
eventually fire, but waiting 10+ minutes is unacceptable UX.
Bundled with the empty error message filter since both handle 429 recovery.
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