Fix UTF-16 supplementary character encoding

[metsw24-max]

This fixes incorrect UTF-16 encoding for supplementary-plane Unicode characters in the UTF-16BE and UTF-16LE transcoder paths.

Changes:

• Fixed low surrogate byte construction in transcoder.cpp
• Added regression tests for:
  • UTF-16BE BMP encoding
  • UTF-16BE supplementary-plane encoding
  • UTF-16LE supplementary-plane encoding

Testing

Added regression coverage in:

 src/test/cpp/helpers/transcodertestcase.cpp