fix: validate reaction message IDs

[vycdev2]

Summary

• reject non-numeric messageId values for the MCP react-to-message tool before Discord lookup
• advertise the numeric constraint in the MCP tool input schema
• add focused request-validation coverage and document the fix

TDD evidence

• Untouched-base RED: node --test tests/mcpMessageValidation.test.mjs failed because the advertised messageId pattern was absent.
• GREEN: the focused test passed after adding matching Zod and JSON-schema constraints.

Verification

• npm run build — passed
• node --test tests/mcpMessageValidation.test.mjs — passed, 2 tests
• npm test — passed, 66 tests
• direct node-pty import probe — passed
• git diff --check upstream/main...HEAD — passed

Risk

Low. Valid numeric Discord message IDs are unchanged; malformed values now fail locally instead of reaching Discord.

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