fix: validate help module selections

[vycdev2]

Summary

• Validate help-module component selections before parsing page and module data.
• Return a friendly recovery embed instead of throwing on malformed interaction data.
• Add focused coverage for valid and malformed selection identifiers.

Verification

• dotnet restore
• dotnet build --no-restore
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter FullyQualifiedName~HelpCommandRegistrationTests
• Full suite: 304 passed, 2 known globalization-invariant baseline failures in NormalizeTimeUntilEventName_NormalizesCase because tr-TR is unavailable in this environment.
• git diff --check passed.
• Repository-wide dotnet format remains blocked by pre-existing line-ending/whitespace findings across the checkout.

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