fix(proxy): classify Anthropic cache_control validation errors as 400, not 502

[steventohme]

What

Anthropic cache_control validation errors (breakpoint overflow past the 4-cap, or a ttl=1h breakpoint ordered after a ttl=5m one) previously fell through ClassifyDispatchError to the generic "Upstream call failed" 502 handler — even though no upstream call was ever attempted. The router's own translate-layer validator (internal/translate/cache_control.go) rejects these before dispatch.

Now classified as a clean 400 (DispatchErrorAnthropicCacheControlInvalid), matching the pattern used for the other client-input error kinds. unwrapToSentinelMessage walks the error's wrap chain to strip the internal "emit body: " prefix while keeping the validator's own dynamic detail ("got 5, maximum is 4").

Why

Found by the router's real-Anthropic smoke suite (#828, smoke/cache_test.go's overflow-rejection scenario) on its first run against the real API — the in-process conformance suite never exercises what ClassifyDispatchError actually returns for this sentinel pair end to end. Split out of #828 per review — that PR is the test harness; this is the standalone production fix it caught.

Testing

• go test ./internal/proxy/... — new unit tests assert 400 + IsClientError() + no leaked wrap prefix for both the overflow and TTL-ordering sentinels.
• go build ./..., go vet ./..., gofmt -l . all clean.

[claude]

Claude finished @steventohme's task —— View job

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• Run git diff to inspect added comment lines
• Analyze comment blocks for verbosity
• Post review if needed

One 4-line godoc block on unwrapToSentinelMessage (lines 229–232 of dispatch_error.go) mostly restates the algorithm — filed as a committable suggestion in the review above.

[workweave-bot]

Advisory only — comment-length nit. Won't block merge.

[greptile-apps]

T-Rex Logs

What T-Rex did

• T-Rex ran targeted proxy-cache-control tests and they exited with code 0.
• T-Rex ran broader proxy package tests and they exited with code 0.
• T-Rex executed the Go toolchain checks (go build, go vet, and gofmt); all commands exited with code 0 and gofmt produced no output.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix(proxy): classify Anthropic cache_con..." | Re-trigger Greptile}

[claude]

Claude finished @steventohme's task —— View job

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• Run git diff to inspect added comment lines
• Analyze comment blocks for verbosity
• Post review if needed

The 3-line godoc on unwrapToSentinelMessage (lines 229–231 of dispatch_error.go) that was flagged in the previous review has already been trimmed to 3 lines. No further comment-length issues found in this revision.

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