fix(websocket): support open diagnostics over h2

[mcollina]

Summary

Fix WebSocket open diagnostics for HTTP/2 connections.

WebSocket over HTTP/2 was already implemented, but subscribing to undici:websocket:open could crash the connection setup because the diagnostics payload assumed the transport always exposed socket.address(). Over HTTP/2 the transport is a stream, so the address needs to be resolved from the underlying session socket.

Changes

• resolve websocket open diagnostic addresses for both HTTP/1.1 sockets and HTTP/2 streams
• make websocket debug logging resilient when address information is unavailable
• add HTTP/2 coverage for websocket handshake diagnostics
• document HTTP/2 websocket handshake response semantics in diagnostics docs

Testing

• npx eslint lib/web/websocket/websocket.js lib/core/diagnostics.js test/websocket/diagnostics-channel-handshake-response-h2.js
• npm run test:websocket

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.91%. Comparing base (4c3a2be) to head (303c5fe).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/core/diagnostics.js	0.00%	6 Missing ⚠️
lib/web/websocket/websocket.js	84.61%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4921   +/-   ##
=======================================
  Coverage   92.90%   92.91%           
=======================================
  Files         112      112           
  Lines       35646    35660   +14     
=======================================
+ Hits        33117    33133   +16     
+ Misses       2529     2527    -2     

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