fix: limit choose command replies

[vycdev2]

Summary

• truncate oversized choose options before adding the response prefix so replies stay within Discord’s 2,000-character limit
• preserve UTF-16 surrogate pairs when truncating and add regression coverage for both the limit and emoji boundary

Verification

• /opt/data/home/.dotnet/dotnet build --no-restore — passed: 0 errors (1 existing SQLite package vulnerability warning)
• /opt/data/home/.dotnet/dotnet test --no-restore --logger 'console;verbosity=minimal' — passed: 430, skipped: 1 globalization-invariant culture test
• python3 -m unittest tools.test_generate_commands_md — passed: 3 tests
• git diff --check — passed

Risk

• Low — only oversized options are shortened, and existing blank-line/whitespace parsing behavior remains covered.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.