fix: route WebSocket upgrades through onRequestUpgrade

[mcollina]

Summary

• deliver WebSocket upgrade responses through the documented onRequestUpgrade handler
• keep legacy onUpgrade handling for compatibility

Fixes #4771.

Testing

• npm test (fails in test:wpt: many existing XHR failures; other suites passed)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 8.57143% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.16%. Comparing base (352c691) to head (f44a6bc).

Files with missing lines	Patch %	Lines
lib/web/fetch/index.js	8.57%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4787      +/-   ##
==========================================
- Coverage   93.25%   93.16%   -0.09%     
==========================================
  Files         109      109              
  Lines       34024    34059      +35     
==========================================
+ Hits        31729    31732       +3     
- Misses       2295     2327      +32     

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