chore: switch deferred promise with Promise.withResolvers()

[trivikr]

This relates to...

N/A

Rationale

The package engine is already >=22.19.0, so Promise.withResolvers() is available in all supported runtimes. This removes an internal deferred helper that is no longer needed and simplifies the affected code paths.

Changes

• Replace createDeferredPromise() call sites with Promise.withResolvers()
• Remove the unused lib/util/promise.js helper
• Update WebSocketStream JSDoc to reflect the built-in deferred return shape
• Update WPT runner code to use Promise.withResolvers() directly

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.95%. Comparing base (48c7cf3) to head (2867c38).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/web/cache/cache.js	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4972      +/-   ##
==========================================
- Coverage   92.96%   92.95%   -0.02%     
==========================================
  Files         111      110       -1     
  Lines       35864    35809      -55     
==========================================
- Hits        33342    33287      -55     
  Misses       2522     2522              

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[mcollina]

lgtm