Add test for http2.server.stream.finish diagnostics channel

[Copilot]

Node.js added the http2.server.stream.finish diagnostics channel in nodejs/node#58560, which fires when ServerHttp2Stream.respond() succeeds. This adds test coverage verifying urllib works correctly with this channel.

• Subscribe to http2.server.stream.finish in an H2 test server scenario
• Make HEAD and GET requests via HttpClient with allowH2: true
• Assert published messages contain valid stream (ServerHttp2Stream), headers (object), and flags (number)

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/node-modules/urllib@740

commit: 6b0e217

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.60%. Comparing base (ca63296) to head (6b0e217).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #740   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files          10       10           
  Lines         723      723           
  Branches      226      226           
=======================================
  Hits          684      684           
  Misses         36       36           
  Partials        3        3           

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