buffer: fix odd UTF-16LE indexOf start

[inoway46]

An odd forward UTF-16LE/UCS2 search start was rounded down, allowing a match before the requested start.
Round it up to the next UTF-16 code-unit boundary instead.

This preserves the existing code-unit-aligned string search semantics. It does not introduce every-byte-offset or cross-code-unit matches, or change Buffer/Uint8Array needle behavior.
Unaligned receivers are tracked separately in #65959.

Fixes: #26448

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (b805fb5) to head (045fc67).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65960   +/-   ##
=======================================
  Coverage   90.17%   90.18%           
=======================================
  Files         771      771           
  Lines      265489   265493    +4     
  Branches    50471    50471           
=======================================
+ Hits       239405   239425   +20     
+ Misses      17042    17007   -35     
- Partials     9042     9061   +19     

Files with missing lines	Coverage Δ
src/node_buffer.cc	69.59% <100.00%> (+0.28%)	⬆️

... and 29 files with indirect coverage changes

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