fix(util): validate UTF-16 surrogate pairs

[efegokdemir]

Summary

Fix UTF8Util.isWellFormed to accept valid non-BMP characters and reject unpaired UTF-16 surrogates.

Changes

• Validate high/low surrogate pairing, including trailing high surrogates.
• Add regressions for valid emoji and malformed surrogate sequences.

Testing

• git diff --check — passed
• Maven module tests — blocked locally because the environment only provides Java 8 while this project requires Java 17; the wrapper also requires an unavailable Java runtime.

Fixes #287