[BUG] mcpb validate warns about icon size even for 512x512 PNGs

[stickerdaniel]

Describe the bug
mcpb validate prints the recommended icon size warning even when the icon is already a valid 512x512 PNG.

To Reproduce

1. Set manifest.json to use a 512x512 PNG icon.
2. Run:

   mcpb validate manifest.json

3. Observe:

   Icon validation passed. Recommended size is 512×512 pixels for best display in Claude Desktop.
   

Expected vs actual behavior
Expected: no size warning for a valid 512x512 PNG.
Actual: the warning is still printed.

Version information
@anthropic-ai/mcpb@2.1.2

Relevant logs or error messages

Manifest schema validation passes!

Icon validation warnings:

  - Icon validation passed. Recommended size is 512×512 pixels for best display in Claude Desktop.