feat: Add readOnly property to FormBuilderDateTimePicker

[CesareIsHere]

This PR adds support for the readOnly property to FormBuilderDateTimePicker, bringing it in line with other form builder fields and Flutter's native TextField behavior.

Changes Made

• Added readOnly property to FormBuilderDateTimePicker constructor with default value false
• Modified the onTap callback to conditionally prevent picker dialogs from opening when readOnly is true
• Updated documentation for the new property

Behavior

When readOnly: true:

• ✅ Field displays the current date/time value normally
• ✅ Date/time picker dialogs do not open when tapping the field
• ✅ Field maintains normal visual appearance (not grayed out like enabled: false)
• ✅ Field remains focusable for accessibility
• ✅ Value is still accessible for form submission and validation
• ✅ Consistent with other FormBuilder fields API

Usage Example

FormBuilderDateTimePicker(
  name: 'created_date',
  readOnly: true,
  initialValue: DateTime.now(),
  decoration: InputDecoration(
    labelText: 'Creation Date',
    hintText: 'This field is read-only',
  ),
)

Use Cases

• Displaying creation/modification timestamps
• Showing calculated dates that shouldn't be modified
• Form sections that are conditionally editable based on user permissions
• Confirmation/review screens where data should be visible but not modifiable

Breaking Changes

None - this is a backward-compatible addition with a sensible default value.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.88%. Comparing base (84dbb3f) to head (b73e503).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1496   +/-   ##
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  Coverage   89.88%   89.88%           
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  Files          21       21           
  Lines         840      840           
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  Hits          755      755           
  Misses         85       85           

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