fix: handle invalid image preprocessing

[vycdev2]

Summary

• Route deepfry-extra, memefy, and blurpify image validation through the existing guarded downscaling helper.
• Return a clear invalid-image response instead of letting corrupt or non-image attachment bytes escape as an unhandled preprocessing exception.

Verification

• dotnet build — passed with 0 errors (2 known dependency vulnerability warnings).
• dotnet test --no-build --filter 'FullyQualifiedName~ImageModuleTests|FullyQualifiedName~ImageResizerTests' --logger 'console;verbosity=minimal' — passed: 3/3.
• dotnet test --no-build --logger 'console;verbosity=minimal' — passed: 428 passed, 1 skipped, 0 failed.
• git diff --check — passed.

Risk

• Low — the change reuses the same guarded preprocessing path already used by the deepfry command and only affects malformed image input handling.

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