ci: fail on clippy warnings

[DCjanus]

Why

Dependency updates can surface warnings that are easy to miss during review.

What

• Make the clippy job fail on warnings.
• Replace the deprecated quick-xml attribute value API with the normalized API.

Evidence

The commits keep the CI gate and warning fix separate.

• 04896bb: only updates the clippy command; the existing quick-xml deprecation warning fails the job: failure.
• fb7f4a9: updates the attribute API; the same clippy job passes: success.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.98%. Comparing base (8598600) to head (fb7f4a9).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   80.98%   80.98%           
=======================================
  Files          23       23           
  Lines        1530     1530           
=======================================
  Hits         1239     1239           
  Misses        291      291           

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