Add return type annotations to TransformableData trait

[cyppe]

This PR adds proper PHPDoc type annotations to the methods in the TransformableData trait to improve static analysis with PHPStan. Specifically:

• Added @return array<string, mixed> to transform(), toArray(), all(), and jsonSerialize() methods
• These annotations accurately reflect that these methods return associative arrays with string keys and mixed values
• Improves type safety and IDE autocompletion support
• Helps PHPStan better understand the return types for static analysis

The changes are purely documentation improvements with no functional impact.

[rubenvanassche]

Thanks!