fix: preserve MCP reaction errors

[vycdev2]

Summary

• Preserve the original Discord API error when an MCP reaction fails and no matching custom guild emoji exists.
• Keep the existing custom-emoji-by-name fallback intact.
• Add a regression test for permission failures on valid Unicode reactions.

Verification

• npm run build — passed
• npm test — passed (66 tests)
• git diff --check — passed

Risk

• Low: the change only replaces a misleading synthesized error with the original Discord failure when custom emoji fallback is unavailable.

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