Exit with non-zero when autofix not applied

[soraxas]

Summary

Closes #409

This PR improves the exit code handling in the fix command when multiple issues are encountered in a single run.

• Refactored the fix command in src/fixit/cli.py to set exit codes more accurately: now, exit code 2 is set for syntax errors, exit code 1 for unfixable violations, and both are combined to 3 if both types of issues are present. This ensures that the CLI communicates the exact result of the operation.
• This is in accordance to 5e49b17

  Return non-zero exit codes for violations/errors

  This tracks exit codes based on the existence of violations and/or
  errors when linting files.

  • 0: everything clean
  • 1: violations only
  • 2: errors only
  • 3: both violations and errors

  Fixes Fixit cli doesn't return non-zero exit code upon exceptions - add --exit-non-zero / --exit-zero options #258

  ghstack-source-id: 2c18d7a
  Pull Request resolved: Return non-zero exit codes for violations/errors #283

Test Plan

• Added a new test method test_fix_exit_codes to verify the exit codes for unfixable violations, syntax errors, and scenarios where both occur together.