fix: prevent race condition between onEnd and onTrailers in HTTP/2 client (#5216)

[mcollina]

Description

Fixes a flaky test test/http2-trailers.js (issue #5216) caused by a race condition between the onEnd and onTrailers event handlers in the HTTP/2 client (lib/dispatcher/client-h2.js).

Root Cause

On Windows, Node.js can emit 'end' before 'trailers' (reversing the expected event order). When onEnd fires first, it calls request.onResponseEnd({}) with empty trailers and removes the 'trailers' listener via releaseRequestStream(stream). By the time onTrailers fires, it sees request.completed === true and returns without passing the actual trailers. The test assertion t.strictEqual(trailers['x-trailer'], 'hello') then fails.

Fix

In onEnd, check stream.listenerCount('trailers') before handling completion:

• If the 'trailers' listener is still registered, defer completion to onTrailers via a process.nextTick fallback instead of finalizing immediately.
• If onTrailers never fires (no-trailers case), the fallback handles completion.
• If onTrailers fires first (normal event order), it removes the 'end' listener and onEnd never runs.

Test Adaptation

The test 'Should remove request-owned http2 stream listeners after completion' now checks listener counts after the process.nextTick fallback has run, since listeners are now removed asynchronously in the no-trailers case.

Testing

All 295 HTTP/2 and client tests pass, including test/http2-trailers.js.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.23%. Comparing base (7235cbb) to head (866df8f).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/dispatcher/client-h2.js	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5343      +/-   ##
==========================================
- Coverage   93.23%   93.23%   -0.01%     
==========================================
  Files         110      110              
  Lines       36631    36688      +57     
==========================================
+ Hits        34154    34206      +52     
- Misses       2477     2482       +5     

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