Add types to MessageBag

[AJenbo]

This PR adds explicit type annotations to the MessageBag class, improving static analysis and overall code clarity.

Changes include:

• Specifying array<string, array<string>> for the internal $messages property.
• Updating docblocks for the constructor and methods like:
  • keys()
  • merge()
  • get()
  • all()
  • transform()
  • messages()
  • getMessages()

These updates help IDEs and static analysis tools better understand the structure of the messages.