fix(client-h2): stop double-decrementing kOpenStreams on stream timeout

[SAY-5]

Fixes #5073.

The HTTP/2 stream timeout handler in lib/dispatcher/client-h2.js does:

stream.on('timeout', () => {
  const err = new InformationalError(`HTTP/2: "stream timeout after ${requestTimeout}"`)
  stream.removeAllListeners('data')
  session[kOpenStreams] -= 1
  if (session[kOpenStreams] === 0) session.unref()
  abort(err)
})

abort(err) ultimately destroys the http2 stream, which fires close, and the close listener a few lines above already decrements kOpenStreams:

stream.once('close', () => {
  stream.removeAllListeners('data')
  session[kOpenStreams] -= 1
  if (session[kOpenStreams] === 0) session.unref()
})

Result: a single timed-out request knocks the counter down twice and it ends at -1 instead of 0. Future stream accounting on the session is then permanently skewed — the === 0 unref check can never match, and reused-session flows that rely on the counter reaching 0 to release the socket start holding sessions open longer than they should.

Leave the decrement to the close handler. The timeout handler only needs to stop the data listener and call abort(); destruction via abort() guarantees close fires exactly once. After this change the repro in the issue reports open streams after timeout: 0 as expected.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.10%. Comparing base (c7a2901) to head (60ba16b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5076      +/-   ##
==========================================
- Coverage   93.10%   93.10%   -0.01%     
==========================================
  Files         110      110              
  Lines       35805    35799       -6     
==========================================
- Hits        33337    33331       -6     
  Misses       2468     2468              

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[mcollina]

Thanks for opening a PR! Can you please add a unit test?

[SAY-5]

Thanks @mcollina — added a regression test in test/http2-timeout.js.

The test drives three sequential bodyTimeout aborts on the same h2 session, then reads the module-local open-streams counter off the session via its Symbol description. With the fix applied the counter returns to 0 after each stream closes; on master (pre-patch) it settles at -3 after the three cycles because the 'timeout' handler and the 'close' handler were both decrementing it.

Verified locally on macOS, Node 25.9:

• npm run lint: clean (eslint --cache)
• node --test --test-isolation=none test/http2-timeout.js: 2/2 pass
• Reverted lib/dispatcher/client-h2.js to master and re-ran the new test to confirm it fails with open-stream counter should be 0 after all streams close, got -3, i.e. the assertion pins the exact drift the fix corrects.