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

Consolidate all response completion and stream cleanup into onRequestStreamClose, which fires on the stream's natural 'close' event (always after 'end' and 'trailers'). Individual event handlers now only store state:

• onEnd: sets state.pendingEnd = true, returns — no releaseRequestStream, no onResponseEnd
• onTrailers: stores trailers in state.trailers, removes 'data' listener, returns — no releaseRequestStream, no onResponseEnd
• onTimeout/onError/onFrameError: abort the request, return — no releaseRequestStream
• onRequestStreamClose: calls releaseRequestStream, then completes the response with state.trailers || {} if pendingEnd && !aborted && !completed

This eliminates the deferredCompletion flag, the process.nextTick fallback, and all complex branching — the stream lifecycle ('end' → 'trailers' → 'close') handles everything regardless of event ordering.

Test Adaptation

Both tests in test/http2-late-data.js now emit 'close' after 'end' to trigger onRequestStreamClose, which is the natural event order in real HTTP/2 streams.

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 93.24%. Comparing base (7235cbb) to head (0967b99).
⚠️ Report is 13 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%   93.24%           
=======================================
  Files         110      110           
  Lines       36631    36678   +47     
=======================================
+ Hits        34154    34200   +46     
- Misses       2477     2478    +1     

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