PICARD-3084: Allow up to 3 char replacement in illegal chars

[knguyen1]

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:
  • Allow up to 3-character replacements (including whitespace) for Windows-illegal characters in the Windows compatibility dialog; relax validation accordingly. Adds unit tests for old vs. new behavior.

Problem

• JIRA ticket (optional): PICARD-3084

• One-for-one replacement and whitespace prohibition limited filename rewriting flexibility (e.g., wanting “: ” → “ - ”).
• UI QLineEdit fields were restricted to length 1, and the validator rejected spaces.

Solution

• UI: Increase maxLength to 3 for all replacement fields in ui/win_compat_dialog.ui and adjust field width for visibility.
• Validation: Update WinCompatReplacementValidator in picard/ui/options/renaming_compat.py to allow multi-character strings and whitespace while still forbidding Windows-illegal characters and directory separators.
• Tests: Add test/test_win_compat_replacement_validator.py:
  • Encodes BEFORE behavior in an in-test OldValidator.
  • Verifies all previously accepted inputs remain accepted.
  • Confirms illegal characters are still rejected.
  • Ensures multi-character and whitespace-only inputs are now accepted.
  • Confirms directory separators remain rejected.
  • Checks validator returns the original (text, pos).

Notes:

• Core replacement logic (replace_win32_incompat) remains unchanged; it already supports string mappings.
• This PR does not add sequence-aware replacements (e.g., specifically mapping “: ” as a pair), which is out of scope.
• Closes: PICARD-3084: Allow up to 3 chars for Windows file name compatibility replacements #2646

Action

Additional actions required:

• Update Picard documentation (picard-docs) to note that Windows-illegal character replacements now accept up to 3 characters (including spaces), and clarify per-character mapping behavior.
• Other (please specify below)

[zas]

LGTM

[phw]

Thanks for finishing this up