fix: validate emoji import selection ids

[vycdev2]

Summary

• Validate emoji import select-menu IDs before parsing them.
• Return an ephemeral error for malformed server or emoji selections instead of allowing the interaction handler to throw.
• Add focused coverage for valid, missing, signed, non-decimal, and overflowing IDs.

Verification

• dotnet restore Morpheus.sln — passed (existing NU1903 SQLite vulnerability warning).
• dotnet build Morpheus.sln --no-restore — passed.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-build --filter FullyQualifiedName~EmojisModuleTests — passed (10 tests).
• Full test suite — 305 passed, 2 failed because the environment is globalization-invariant and has no ICU package for the existing Turkish-culture tests.
• git diff --check — passed.

Risk

• Low: only malformed interaction payload handling changes; valid decimal Discord IDs retain existing behavior.

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