http2: optimize final response writes

[GetThatCookie]

This PR is one of four focused changes split out of #64980 following review
feedback.

The full context, rationale, related work, benchmarks, and validation details
are documented there.

The AI-assistance disclosure in #64980 applies to this split PR as well.

[nodejs-github-bot]

Review requested:

• @nodejs/http2
• @nodejs/net
• @nodejs/performance

[codecov]

Codecov Report

❌ Patch coverage is 79.31034% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (f00fb75) to head (bea785c).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/http2/compat.js	79.31%	6 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #64989 +/- ##
=======================================
Coverage 90.27% 90.27% =======================================
Files 762 762 Lines 247534 247556 +22 Branches 46694 46693 -1 =======================================
+ Hits 223457 223486 +29 + Misses 15529 15505 -24 - Partials 8548 8565 +17 

Files with missing lines	Coverage Δ
lib/internal/http2/compat.js	96.71% <79.31%> (-0.51%)	⬇️

... and 33 files with indirect coverage changes

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[GetThatCookie]

Closing this because the isolated HTTP/2 change did not show a meaningful end-to-end benefit.