src: fix crash when writing odd-length hex string via Writev

[RajeshKumar11]

Summary

Writing an odd-length hex string to a stream that batches writes via
Writev (e.g. http.request which auto-corks its socket) would
fatal-assert:

node: ../src/string_bytes.cc: Assertion `str->Length() % 2 == 0 &&
"invalid hex string length"' failed.

Reproduction:

const http = require('http');
const req = http.request('http://example.org', { method: 'POST' });
req.write('1', 'hex'); // crashes

Root cause

StringBytes::StorageSize had a CHECK that asserted the hex string
length was even. When writes are batched through Writev, this check
runs before any data is written. A single Write (non-corked path)
did not crash because StringBytes::Write calls HexDecode, which
silently drops the trailing incomplete nibble.

Fix

Remove the CHECK and use integer division for the HEX case, which
naturally rounds down for odd lengths. This is already the behaviour
of StringBytes::Size and HexDecode.

Test

Added test/parallel/test-http-odd-hex-write.js with three cases:

1. HTTP POST with a single odd-length hex write (was crashing)
2. HTTP POST with cork/uncork and mixed hex writes (was crashing)
3. net socket with cork/uncork and an odd-length hex write

Fixes: #45150

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.34%. Comparing base (30a7e28) to head (0e3192c).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63658      +/-   ##
==========================================
- Coverage   91.95%   90.34%   -1.61%     
==========================================
  Files         379      732     +353     
  Lines      166454   236434   +69980     
  Branches    25427    44532   +19105     
==========================================
+ Hits       153058   213614   +60556     
- Misses      13104    14522    +1418     
- Partials      292     8298    +8006     

Files with missing lines	Coverage Δ
src/string_bytes.cc	71.54% <ø> (ø)

... and 482 files with indirect coverage changes

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