fix AR mime-type reader error wrap to preserve errors.Is identity

[johnelliott]

Summary

Same-shape follow-up to PR #4929. Switch the inner %v to %w at pkg/handlers/ar.go:101 so the error returned from newMimeTypeReader stays inspectable via errors.Is after being wrapped with ErrProcessingWarning.

Why

Reachability re-audit confirms the path:

bufReader.Peek -> io.SectionReader over the deb library reader
              -> BufferedReadSeeker.ReadAt
              -> the original io.Reader passed to HandleFile

For source readers backed by HTTP/GCS-style transports, parent-context cancellation surfaces here as a Read error whose chain satisfies errors.Is(err, context.DeadlineExceeded) (or context.Canceled). The %v wrap strips that inner identity, so isFatal at pkg/handlers/handlers.go:482 falls into the ErrProcessingWarning branch and classifies the cancelled/deadline-exceeded read as a non-fatal warning.

Blast radius is lower than the sibling ar.go:109 fix because processARFiles checks ctx.Done() at the top of each loop iteration, so the next iteration catches the cancellation anyway. This change is a symmetry/correctness fix that lets the classifier act on the actual cause rather than the outer warning sentinel.

What changed

• pkg/handlers/ar.go:101: %v -> %w on the inner error.
• pkg/handlers/ar_test.go: regression test TestARHandler_MimeTypeReaderErrPreservesIdentity that drives processARFiles directly with a crafted *deb.Ar whose first entry's data section returns the target error from ReadAt. Table-driven over context.DeadlineExceeded, context.Canceled, and io.ErrUnexpectedEOF (io.EOF is filtered inside newMimeTypeReader and never reaches the wrap site). Each case asserts the outer ErrProcessingWarning wrap is preserved, the inner cause is inspectable via errors.Is, and isFatal classifies based on the inner cause.

Test plan

• go test ./pkg/handlers/ -count=1 passes
• Reverting the fix to %v makes the new test fail on errors.Is(warnErr, tc.innerErr) and isFatal for the fatal cases
• golangci-lint clean on the changed package

---

Note

Low Risk
Small error-wrapping change plus targeted tests; behavior change is limited to how AR entry read failures are classified via errors.Is.

Overview
Adjusts AR archive handling to wrap newMimeTypeReader failures with %w (instead of %v), preserving the underlying error in the errors.Is chain when emitting ErrProcessingWarning.

Adds a regression test that drives processARFiles with a crafted deb.Ar reader that injects specific ReadAt errors, asserting the inner error remains detectable via errors.Is and that isFatal classifies context cancellation/deadlines as fatal while treating io.ErrUnexpectedEOF as non-fatal.

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