fix: handle all HTTP/2 request stream sync errors

[mcollina]

Summary

• handle every synchronous session.request() failure in the HTTP/2 dispatcher via Undici's normal request error path
• preserve the original Node.js error as the InformationalError cause
• add a regression test for ERR_HTTP2_HEADER_SINGLE_VALUE from duplicate single-value HTTP/2 headers and verify queued requests resume

Fixes #4356

Testing

• npx borp -p "test/http2-invalid-connection-headers.js"
• npx eslint lib/dispatcher/client-h2.js test/http2-invalid-connection-headers.js
• npm run test:h2:core

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (81fdf87) to head (77541ab).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5311   +/-   ##
=======================================
  Coverage   93.23%   93.24%           
=======================================
  Files         110      110           
  Lines       36574    36570    -4     
=======================================
- Hits        34101    34099    -2     
+ Misses       2473     2471    -2     

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