http2: fix out-of-bounds write in altsvc frame buffers

[Nashit-h]

Http2Session::AltSvc sizes the origin and value byte buffers to the exact string length with MaybeStackBuffer, then fills them via WriteOneByteV2 with String::WriteFlags::kNullTerminate, which writes length + 1 bytes for the trailing NUL. Once origin or alt is longer than the 1024-byte inline stack storage the buffer is heap-allocated at exactly the string length, so that NUL lands one byte past the end, a heap-buffer-overflow that ASAN flags on session.altsvc('h2=":8000"; ma=' + '0'.repeat(2000), streamId). The terminator is never needed because the buffers reach nghttp2_submit_altsvc with explicit origin_len and value_len, so drop kNullTerminate and let each write match its allocation.

[nodejs-github-bot]

Review requested:

• @nodejs/http2
• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (cb9bb66) to head (f66898b).
⚠️ Report is 291 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65216      +/-   ##
==========================================
- Coverage   90.31%   90.31%   -0.01%     
==========================================
  Files         760      760              
  Lines      248532   248529       -3     
  Branches    46908    46897      -11     
==========================================
- Hits       224467   224463       -4     
+ Misses      15505    15504       -1     
- Partials     8560     8562       +2     

Files with missing lines	Coverage Δ
src/node_http2.cc	81.77% <100.00%> (-0.03%)	⬇️

... and 22 files with indirect coverage changes

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[pimterry]

@Nashit-h linters hadn't run before I approved, and unfortunately they're failing, can you take a look? You can run locally with make lint, and then push a new commit to fix them and they'll be squashed together when this lands.