fix: encode Urban Dictionary search terms

[vycdev2]

Summary

• Percent-encode Urban Dictionary command terms before placing them in the API query string.
• Preserve the random-definition endpoint for null, empty, and whitespace-only input.
• Add regression coverage for reserved query characters and missing terms.

Verification

• dotnet build — passed with 0 errors (2 existing NU1903 vulnerability warnings for SQLitePCLRaw.lib.e_sqlite3 2.1.6).
• dotnet test --no-build — passed all 192 tests.
• python3 tools/generate_commands_md.py — ran successfully; generated path-separator-only differences were discarded because command metadata did not change.
• git diff --check — passed.

Risk

• Low: the change is limited to encoding one outbound query value and preserving the prior fallback endpoint.

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