ci: upgrade web test runners

[lyzno1]

Summary

• Upgrade the web test workflow jobs from depot-ubuntu-24.04 to depot-ubuntu-24.04-4
• Restore the 4 CPU / 16 GB runner class for Vitest shards, report merging, and dify-ui browser-mode tests

Testing

• git diff --check -- .github/workflows/web-tests.yml
• commit hook ran eslint --fix on the staged workflow file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.64%. Comparing base (818a71d) to head (58a14f1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35593      +/-   ##
==========================================
- Coverage   85.64%   85.64%   -0.01%     
==========================================
  Files        4454     4454              
  Lines      208888   208890       +2     
  Branches    39056    39058       +2     
==========================================
  Hits       178897   178897              
- Misses      26845    26847       +2     
  Partials     3146     3146              

Flag	Coverage Δ
dify-ui	93.01% <ø> (ø)
web	86.48% <ø> (-0.01%)	⬇️

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