fix(combo): fall back on Responses SSE failures

[rushsinging]

Summary

• reject Responses-compatible SSE streams that report an upstream failure before useful output
• let the existing Combo flow advance to the next model instead of treating the HTTP 200 stream as successful
• preserve the current no-fallback behavior after useful output starts, avoiding duplicate partial output

The detector covers response.failed events, event:error envelopes, top-level error payloads, and nested failed/error response states. It reports a generic validation reason and does not expose the upstream error message.

Evidence

• 29 passing, 0 failing across the focused and adjacent Combo response-quality unit tests
• npm run typecheck:core — passed
• targeted ESLint checks — passed

No docs/superpowers files are included.

[gemini-code-assist]

Code Review

This pull request introduces upstream error detection for SSE streams in validateResponseQuality, allowing the combo service to failover early when encountering pre-content errors. It also adds comprehensive unit tests in tests/unit/ to validate this behavior. The review feedback correctly identifies a potential runtime TypeError in the new isStreamingUpstreamError helper if the parsed payload is not a plain object, and provides a robust code suggestion to safely guard the property checks.

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[rushsinging]

Superseded by #7256, which ports the same fix onto release/v3.8.49.