fix(http2): avoid double-decrementing open streams on timeout

[trivikr]

This relates to...

Fixes: #5073

Rationale

client-h2.js was decrementing session[kOpenStreams] in both the timeout handler and the stream close handler.

Since the timeout path aborts and closes the stream, a single timed-out request could underflow the counter and skew H2 session ref/unref bookkeeping.

Changes

• Remove the extra session[kOpenStreams] decrement from the HTTP/2 stream timeout handler.
• Keep stream accounting single-sourced in the stream close handler.

Features

N/A

Bug Fixes

Prevent kOpenStreams from being decremented twice for a single timed-out HTTP/2 request.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.10%. Comparing base (f6c5dda) to head (fd2a716).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5074   +/-   ##
=======================================
  Coverage   93.10%   93.10%           
=======================================
  Files         110      110           
  Lines       35799    35793    -6     
=======================================
- Hits        33329    33325    -4     
+ Misses       2470     2468    -2     

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[trivikr]

Merged in #5076