fix: Implement validation for extension_data field in FRProject

[harshith1118]

Summary

This PR addresses the unimplemented validation for the `extension_data` field in `FRProject`
  class that was previously commented out with a TODO.

 ## Changes Made
 1. Implemented the validation in `FRProject.__post_init__()` method that raises `
  MANIP_ThanksError` when `extension_data` is not an empty dictionary `{}`.
 2. Uncommented and updated the corresponding test in `test_FRProject_post_init()` to verify this
  validation works properly.

 ## Why this change is needed
 - The validation was previously commented out with a TODO: `# TODO #if self.extension_data !=
  {}: raise MANIP_ThanksError() # also uncomment test`
- The code comment states: "I could not find out what it would be used for, seems to be always
  {}"
- This ensures projects maintain the expected format where `extension_data` should always be
  empty

## Verification
- Normal project creation with empty extension_data still works
- Projects with non-empty extension_data now properly raise MANIP_ThanksError
- Import of modified modules works without syntax errors
3. Submit the pull request to merge your fix-extension-data-validation branch into the main branch of the
  original repository