Write HTTP/2 BytesBody responses directly

[quinnj]

Summary

• fast-path HTTP/2 server responses backed by BytesBody by writing unread backing bytes directly
• close BytesBody responses in a finally, preserving the streaming-body cleanup behavior
• add HTTP/2 server coverage for a partially consumed BytesBody response

Notes

This hardens the path implicated by #1271. I could not reproduce the reported linear growth locally on macOS/aarch64 or native Linux/arm64, but this removes the per-read streaming-body chunk path for buffered BytesBody responses while preserving cursor/close semantics.

Verification

• julia --project=. test/http2_server_tests.jl
• local 24k-request H2 repro in BytesBody mode stayed flat
• Docker native Linux/arm64 24k-request H2 repro in BytesBody mode stayed flat

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.93%. Comparing base (b93ad95) to head (2407cb2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1272      +/-   ##
==========================================
+ Coverage   84.51%   84.93%   +0.41%     
==========================================
  Files          28       28              
  Lines       10696    10711      +15     
==========================================
+ Hits         9040     9097      +57     
+ Misses       1656     1614      -42     

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