Update vitest to v4

[kachkaev]

Closes https://nvd.nist.gov/vuln/detail/CVE-2025-62522 in transient vite dependency (which got an update).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.37%. Comparing base (34426aa) to head (2cf1cb8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   96.56%   95.37%   -1.20%     
==========================================
  Files           6        5       -1     
  Lines         233      108     -125     
  Branches       41       24      -17     
==========================================
- Hits          225      103     -122     
+ Misses          8        5       -3     

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[kachkaev]

Method of counting code coverage has changed, so a failure in https://github.com/gicentre/prettier-plugin-elm/pull/67/checks?check_run_id=55087889209 is unsurprising.