Rewrites the stale branch on top of current `main` and preserves the original issue as regression coverage for the exact OpenRouter JSON 404 payload from #51571.
No production behavior changes are introduced here; current `main` already classifies this payload as `model_not_found`, and this merge locks that in across the shared matcher, failover classifier, and fallback loop.
Co-authored-by: 屈定 <mrdear@users.noreply.github.com>
Co-authored-by: Altay <altay@uinaf.dev>
屈定
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