fix: forward the field error state to the checkbox in FormBuilderCheckbox #1503

[Yusufihsangorgel]

Closes #1503

FormBuilderCheckbox never told the checkbox about the field's error state, so WidgetStateBorderSide.resolveWith / fillColor resolving WidgetState.error had no effect, as reported in the issue.

The builder now passes isError: state.hasError to the internal CheckboxListTile, which is the Material way to put the checkbox itself into the error state, so any side, fillColor or checkbox theme that resolves WidgetState.error starts working without new API surface.

Added a widget test: the checkbox reports isError after a failed validate and drops it once the value is valid again. flutter test and flutter analyze are clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.36%. Comparing base (f8706f7) to head (8adcbce).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1515   +/-   ##
=======================================
  Coverage   92.35%   92.36%           
=======================================
  Files          21       21           
  Lines         863      864    +1     
=======================================
+ Hits          797      798    +1     
  Misses         66       66           

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