fix(combo): avoid false ALL_ACCOUNTS_INACTIVE on quality failures

[CruxExperts]

Summary

• prevent false ALL_ACCOUNTS_INACTIVE terminal errors when an upstream 200 response fails combo quality validation
• propagate explicit terminal context (lastStatus=502, quality-validation lastError) in both priority and round-robin combo paths
• add regression tests covering single-target quality-validation failures for priority and round-robin strategies

Why

In v3.7.2 combo routing, a 200 response that fails validateResponseQuality could exit the loop without terminal status context. In single-target scenarios this fell through to the generic ALL_ACCOUNTS_INACTIVE 503, which is misleading and can trigger false outage signals.

Validation

• node --import tsx/esm --test tests/unit/combo-routing-engine.test.ts
• local hook suite passed during commit
• targeted regression tests added in tests/unit/combo-routing-engine.test.ts

Related

• Closes fix(combo): false ALL_ACCOUNTS_INACTIVE when single-target response fails quality validation #1707

[gemini-code-assist]

Code Review

This pull request updates the combo routing logic to provide more accurate error messages and status codes when upstream responses fail quality validation. By capturing the specific quality failure reason and defaulting to a 502 status code, the system avoids misleading 'ALL_ACCOUNTS_INACTIVE' errors. The changes include updates to both priority and round-robin strategies, along with new unit tests in the appropriate directory. I have no feedback to provide.

[diegosouzapw]

Thanks @CruxExperts for identifying this quality validation gap! 🎉 Your fix was spot-on and the regression tests you added are a great addition. We had already applied the production fix independently, but your tests now lock in the correct behavior. Merged into release/v3.7.3.