fix: ignore invalid Minecraft favicons

[vycdev2]

Summary

• Decode Minecraft server favicons without assuming every API value contains a data-URI comma.
• Skip missing, empty, or malformed favicons so pingmc can still return the server-status embed.
• Cover valid data URIs, raw Base64, and malformed values with regression tests.

Verification

• dotnet build --no-restore — passed with one existing SQLite package vulnerability warning.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter 'FullyQualifiedName~TryDecodeMinecraftFavicon' --verbosity minimal — passed (7 tests).
• dotnet test --no-restore --verbosity minimal — 304 passed, 2 failed because this environment runs in .NET globalization-invariant mode and cannot load the existing tr-TR tests (tracked by test: skip unavailable culture regression #81).
• git diff --check — passed.

Risk

• Low: valid favicon attachments are preserved; invalid favicon data now falls back to the existing status embed without an attachment.

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