prepareForEncoding method

Prepares the tag for synchronous encoding by loading any async data.

For most tags, this returns the tag unchanged. Tags with async data (like artwork) should load their data and return a tag with immediate data available for synchronous encoding operations.

This method is only called if requiresAsyncPreparation returns true. The default implementation returns the tag unchanged, which is appropriate for tags that don't require async preparation.

@returns A future that completes with a tag ready for synchronous encoding