fix nested archive handler error wraps to preserve errors.Is identity

[johnelliott]

Summary

Two same-shape %v error wraps in nested archive handlers drop the inner
error's identity from the errors.Is chain, causing the
isFatal classifier in pkg/handlers/handlers.go to misclassify
cancellation and deadline-exceeded conditions as non-fatal warnings.
This is the identical bug fixed for the canonical chunk-reader wrap in
PR #4929.

Affected sites:

• pkg/handlers/ar.go:109
• pkg/handlers/rpm.go:120

Both wrap the return value of handleNonArchiveContent, which can be
ctx.Err() (context.Canceled / context.DeadlineExceeded) when its
internal CancellableWrite fails. The wrapped error feeds
isFatal at pkg/handlers/handlers.go:425. With %v, neither
errors.Is(err, context.Canceled) nor
errors.Is(err, context.DeadlineExceeded) matches, so the classifier
falls through to the ErrProcessingWarning case and processing
continues despite a fatal cause.

Fix

Change the inner %v to %w in both wraps, matching the shape used by
PR #4929. Multi-%w in a single fmt.Errorf is supported on Go
1.20+; this repo is on Go 1.24.

Behavior change

Cancelled or deadline-exceeded processing inside an AR or RPM nested
archive now correctly terminates instead of being logged as a warning
and continuing.

Tests

Added a regression test for each handler:

• TestARHandler_NonArchiveErrPreservesIdentity in ar_test.go
• TestRPMHandler_NonArchiveErrPreservesIdentity in rpm_test.go

Each test injects a chunk reader that cancels the parent context before
delivering its chunk, forcing handleNonArchiveContent's
CancellableWrite to return context.Canceled, which the surrounding
handler wraps. The tests assert the outer ErrProcessingWarning wrap is
preserved, the inner cancellation cause is inspectable via
errors.Is, and isFatal classifies the wrapped error as fatal.

Verified the tests catch the regression by reverting just the wrap
fix: both tests fail on errors.Is(err, context.Canceled) and
isFatal(err), then pass again once the fix is reapplied.

Test plan

• go test ./pkg/handlers/ -count=1 passes
• go vet ./pkg/handlers/ clean
• Catches-the-bug verification: revert %w -> %v -> tests fail; reapply -> tests pass

---

Note

Low Risk
Small, targeted change to error wrapping plus new tests; behavior only changes for cancellation/deadline paths in AR/RPM processing and is unlikely to impact normal success flows.

Overview
Fixes AR and RPM nested archive handlers to wrap handleNonArchiveContent failures with %w (instead of %v) while still tagging them with ErrProcessingWarning, preserving errors.Is identity for underlying causes like context.Canceled/context.DeadlineExceeded.

Adds regression tests (TestARHandler_NonArchiveErrPreservesIdentity, TestRPMHandler_NonArchiveErrPreservesIdentity) that inject a cancelling chunk reader and assert both the outer warning wrap and the inner cancellation cause remain detectable and are classified as fatal by isFatal.

^{Reviewed by Cursor Bugbot for commit 7b66333. Bugbot is set up for automated code reviews on this repo. Configure here.}

[johnelliott]

Question on the test assertions: any reason these use assert.True(t, errors.Is(warnErr, target), "...") rather than assert.ErrorIs(t, warnErr, target, "...")?

The testify helper produces a better diagnostic on failure (it prints the actual error chain), and we already import github.com/stretchr/testify/assert. Same applies to the matching test in rpm_test.go. Happy to leave as-is if there's a reason; just wanted to ask.

[rosecodym]

left some style notes