#22589: fix: trigger failover on gateway websocket connection errors

## Summary - Treat gateway/websocket transport disconnects as timeout failover signals. - Cover common transport failures such as "Connection error", "gateway not connected", websocket disconnect text, and relevant gateway close codes. - Add regression tests to ensure these errors map to timeout failover. ## Why Issue #22576 reports that gateway↔cloud websocket disconnects show up as connection errors but do not trigger model failover. This change routes those transport failures into the existing timeout failover path. ## Validation - `pnpm -s vitest run -c vitest.e2e.config.ts src/agents/pi-embedded-helpers.isbillingerrormessage.e2e.test.ts`