fix(h2): fix flaky stream end handling on macOS

[mcollina]

Summary

• Fixes flaky Dispatcher#Connect test on macOS in test/http2-dispatcher.js
• The 'end' event handler was incorrectly treating stream state 6 (HALF_CLOSED_REMOTE) as an error based on timing-dependent state checks
• Added a responseReceived flag to properly distinguish between normal completion and error cases

Problem

The test was failing intermittently on macOS with:

Error [InformationalError]: HTTP/2: stream half-closed (remote)

The root cause was a stream state check stream.state.state < 6 that excluded state 6 (HALF_CLOSED_REMOTE). However, this is a valid state when the server finishes sending its response. Timing differences across platforms caused the stream to be in state 6 when 'end' fired, triggering a false error.

Fix

Instead of checking stream state (which is timing-dependent), track whether we received a response:

• Added responseReceived flag set to true when 'response' event fires
• In 'end' handler: if response was received → complete normally
• If no response was received (e.g., server destroyed stream before sending headers) → throw the "half-closed" error

This properly differentiates between:

1. Normal completion: Server sends response then closes stream
2. Error case: Server destroys stream without sending a response

Test plan

• Dispatcher#Connect test passes (the originally flaky test)
• Should throw informational error on half-closed streams (remote) passes (error case still works)
• All H2 tests pass locally

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (4920fc4) to head (a5d2c02).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4762      +/-   ##
==========================================
- Coverage   93.26%   93.26%   -0.01%     
==========================================
  Files         109      109              
  Lines       34029    34023       -6     
==========================================
- Hits        31738    31732       -6     
  Misses       2291     2291              

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