Fix: add validation for config.json

[SyedaAnshrahGillani]

This PR introduces the following changes to prevent such crashes and provide
clearer feedback to the user:

• Safe Key Access: Uses the .get() method to safely retrieve values from the
  configuration dictionary, preventing KeyError exceptions.
• Presence Validation: Explicitly checks for the required keys (type,
  model_file, and voices) and raises a ValueError with a clear, informative
  message if any are missing.
• Improved Error Messaging: Enhances the existing error message for an
  unsupported model type to be more specific and helpful for debugging.

These changes make the model loading logic more resilient to invalid
configurations and improve the overall user experience by providing
actionable error messages.