Add ButtonStyle customization for OK and Cancel buttons

[rydmike]

This commit adds support for custom ButtonStyle properties to the ColorPickerActionButtons class, allowing developers to fully customize the appearance of the OK and Cancel dialog action buttons.

New properties:

• dialogOkButtonStyle: Optional ButtonStyle for the OK button
• dialogCancelButtonStyle: Optional ButtonStyle for the Cancel button

These styles override the default theme styling for the button type, enabling customization of foreground color, background color, padding, elevation, shape, and other button style properties.

This addresses issue #95 by providing a flexible way to customize button appearance without needing to wrap the picker in a Theme widget.

Usage example:

ColorPickerActionButtons(
  dialogOkButtonType: ColorPickerActionButtonType.elevated,
  dialogOkButtonStyle: ElevatedButton.styleFrom(
    foregroundColor: Colors.white,
    backgroundColor: Colors.blue,
  ),
  dialogCancelButtonStyle: TextButton.styleFrom(
    foregroundColor: Colors.red,
  ),
)

Tests: Added comprehensive unit tests for ButtonStyle functionality

---

Note

Adds optional dialogOkButtonStyle and dialogCancelButtonStyle to ColorPickerActionButtons and applies them to dialog buttons.

• Models (lib/src/models/color_picker_action_buttons.dart):
  • Add dialogOkButtonStyle and dialogCancelButtonStyle ButtonStyle? properties.
  • Update copyWith, ==, hashCode, and debugFillProperties to include new fields.
• UI (lib/src/color_picker.dart):
  • Apply actionButtons.dialogOkButtonStyle/dialogCancelButtonStyle to all dialog button variants (TextButton, OutlinedButton, ElevatedButton, FilledButton, FilledButton.tonal).
• Tests (test/color_picker_action_buttons_test.dart):
  • Add comprehensive tests for the new style properties (set/get, copyWith, equality/inequality, null-retain behavior).
  • Relax toString test to check key substrings and update constants where appropriate.

^{Written by Cursor Bugbot for commit 381e44e. This will update automatically on new commits. Configure here.}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.06%. Comparing base (63b97eb) to head (381e44e).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   90.95%   91.06%   +0.11%     
==========================================
  Files          24       24              
  Lines        2354     2384      +30     
==========================================
+ Hits         2141     2171      +30     
  Misses        213      213              

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