perf(http2): reduce writeH2 per-request callback allocations

[trivikr]

This relates to...

Reduces allocation churn in the HTTP/2 request write path.

Rationale

writeH2 created a few request-local cleanup/error callbacks that did not need to capture request-local state.
Under high concurrency, these extra closures add avoidable allocation pressure.

Changes

Hoists shared HTTP/2 stream cleanup handlers and uses per-stream symbol state only where needed.

Features

N/A

Bug Fixes

N/A

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

Assisted-by: openai:gpt-5.5

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (aeed21a) to head (90f2fdd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5138   +/-   ##
=======================================
  Coverage   93.15%   93.16%           
=======================================
  Files         110      110           
  Lines       36212    36234   +22     
=======================================
+ Hits        33735    33757   +22     
  Misses       2477     2477           

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[mcollina]

lgtm