fix: update free text validation regex handling

[ibrahimozkn]

This pull request improves the validation and user experience for custom free-text questions by simplifying error messages, ensuring consistent regex validation, and refactoring related code in both the app and the designer. The changes focus on making custom input validation more robust and user-friendly, especially for fields requiring custom regular expressions.

Localization and Error Message Simplification:

• Changed the free_text_custom_error localization in both English and German to a simpler, more general message, and removed the {pattern} placeholder from all relevant files (app_en.arb, app_de.arb, app_localizations.dart, app_localizations_en.dart, app_localizations_de.dart). [1] [2] [3] [4] [5]

App-Side Free-Text Validation Refactor:

• Updated the validation logic in free_text_question_widget.dart to use a new helper (free_text_regex_validation.dart) for building and validating full-match regex patterns, and to use the simplified error message when validation fails. [1] [2] [3] [4]

Designer-Side Regex Validation Improvements:

• Enhanced the QuestionFormViewModel in the designer to validate custom regex expressions more robustly, ensuring invalid patterns are caught early, and to update related controls when the free-text type changes. [1] [2] [3]
• Updated the free-text question form view to display appropriate validation messages for required fields and invalid patterns.

[johannesvedder]

1. Complete is still possible ven though text is not in required format. Is this fixable?

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Visit the preview URL for this PR (updated for commit df995b5):

• https://studyu-dev-app--pr795-fix-freetextfield-va-7qi1z252.web.app
• https://studyu-dev-designer--pr795-fix-freetextfield-va-m9r7hyz0.web.app

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