fix(combo): lock a model after a local target timeout

[HouMinXi]

Summary

• A combo's own per-target timer returns 504 combo_target_timeout. That code is request-scoped, so the provider circuit breaker stays closed. The model-lockout checks used the same flag and skipped the model too.
• After this change, that timeout records a model lockout. The next request does not spend another gate wait on the same model. Context-length and other request-scoped failures still do not lock the model. Provider cooldown is unchanged.
• Related to fix(resilience): Combo gate timeout (synthetic 504) never triggers Model Lockout; stale TURN_GUARD entry (120s TTL) blocks retries with 409 #15414

Validation

• node --import tsx/esm --test tests/unit/combo/combo-target-timeout-standards.test.ts tests/unit/combo-model-lockout-honors-reset-1308.test.ts tests/unit/combo/combo-target-exhaustion.test.ts — 68 pass, 0 fail
• Replacing the timeout exception with "never lock when request-scoped" makes the new timeout test fail and leaves the context-length test passing
• Three LOCAL forge runs on the uncommitted diff, backend review-default. Each ended PENDING with 0 CONFIRMED. The UNCERTAIN notes restated the lockout branch (first-failure vs exhausted-retry, and whether every request-scoped failure locks). They did not show a missed timeout or a locked context-length failure. The tests cover those two cases.

Tests Added Or Updated

• tests/unit/combo/combo-target-timeout-standards.test.ts
  • typed combo_target_timeout stays request-scoped for the provider breaker and still records a model lockout
  • context_length_exceeded still does not record a model lockout

Coverage Notes

• The new tests call the decision function. They do not run a live combo against a slow upstream.

Reviewer Notes

• Round-robin handles its own failures and does not call executeTargetAttempt. This change covers the reset-aware path named in the report, not round-robin.
• The turn-in-progress guard has no 120s TTL. Release is on the chat finally path and on stream cancel. That part of the report was not changed.