chore: exclude benchmark files from coverage reports

[johncarmack1984]

With benchmarks living next to the code as src/**/*.bench.ts, the coverage config picks them up as source. They only run under vitest bench, which collects no coverage, so they can never report anything but 0% — every PR that adds one gets a red patch-coverage comment that no test can fix (#8127 read 0% patch with 88 lines "missing" for exactly this reason, and the bench file is all that keeps #8158's comment red today). This excludes them from the unit and integration coverage reports the same way **/*.test.ts already is, so the patch number reflects testable code again.

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Confirm you have read our AI policy here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (2a940c1) to head (fea066f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8159      +/-   ##
==========================================
+ Coverage   93.28%   93.93%   +0.65%     
==========================================
  Files         301      290      -11     
  Lines       25081    24905     -176     
  Branches     6578     6569       -9     
==========================================
- Hits        23397    23395       -2     
+ Misses       1684     1510     -174     

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