Support server-side pagination in SfDataGrid column filters

[ariefwijaya]

Summary

This PR contains several related improvements so SfDataGrid's built-in filters work correctly in server-side pagination scenarios, where DataGridSource.rows only holds one page of data at a time:

1. CheckboxFilterValueProvider mixin — A DataGridSource can opt into this mixin to supply a static list of values for the checkbox filter popup per column, instead of the default row-derived list.
2. Equals/DoesNotEqual use text input — Adds equals and doesNotEqual to textFieldFilterTypes in DataGridAdvancedFilterHelper, enabling free-text input for these filter types in the advanced filter popup.
3. advancedFilterType on FilterPopupMenuOptions — Exposes a new advancedFilterType field so callers can override the filter mode (text, numeric, date) per column without relying on cell-value inference.
4. AdvancedFilterType exported from public API — Re-exports AdvancedFilterType from the package's public barrel so consumers can reference the enum without reaching into internal paths.
5. Fix null-value type-check crash — Guards _debugCheckDataType so that filter conditions with a null value (i.e. "Is Null" / "Is Not Null" filter types) no longer throw a FlutterError when rows are refreshed or a column is sorted.

Motivation

When using server-side pagination, DataGridSource.rows only contains one page of data at a time. This causes multiple problems with the built-in filter UI:

Checkbox filter:
CheckboxFilterView derives its unique values from DataGridSource.rows, so values from other pages disappear from the filter list on navigation, making the checkbox filter unusable for server-side grids. The CheckboxFilterValueProvider mixin lets the source provide the full value set explicitly.

Advanced filter (equals/doesNotEqual):
equals and doesNotEqual were only driven by a dropdown populated from the current page's rows. Values from other pages are not available, making exact-match filtering impractical. Users who know the exact value they want should be able to type it directly instead of selecting from an incomplete list.

Advanced filter (column type inference):
For server-side grids, cell values may be stored as strings regardless of the logical column type (date, numeric). The automatic AdvancedFilterType inference based on cell-value type then produces the wrong filter UI. FilterPopupMenuOptions.advancedFilterType lets the caller set the correct filter mode explicitly.

Null/Not Null filter crash:
Applying an "Is Null" / "Is Not Null" filter on a date or numeric column stores null as the condition value. On a subsequent data refresh or sort, _debugCheckDataType compared null's runtime type against the column's expected type and threw a FlutterError, crashing the grid.

Usage

CheckboxFilterValueProvider

class MyDataSource extends DataGridSource with CheckboxFilterValueProvider {
  @override
  List<Object>? getCheckboxFilterValues(String columnName) {
    if (columnName == 'status') return <Object>['Active', 'Inactive'];
    return null; // fall back to default (row-derived) behaviour
  }
}

advancedFilterType on FilterPopupMenuOptions

GridColumn(
  columnName: 'trx_date',
  filterPopupMenuOptions: const FilterPopupMenuOptions(
    advancedFilterType: AdvancedFilterType.date,
  ),
  label: const Text('Date'),
)

Test plan

• Checkbox filter popup shows all custom values on any page of a server-side paginated grid.
• Returning null from getCheckboxFilterValues falls back to default row-derived values.
• Sources that do not use the mixin are unaffected.
• Selection state: values present in effectiveRows are checked when a filter is active; all values are checked when no filter is active.
• Selecting equals / doesNotEqual in the advanced filter popup shows a text input instead of a dropdown, and filtering by a typed value works.
• Other advanced filter types (begins with, ends with, contains, etc.) are unaffected.
• advancedFilterType on FilterPopupMenuOptions forces the popup to text/numeric/date mode.
• AdvancedFilterType is importable from the public package barrel.
• Applying "Is Null" / "Is Not Null" on a date column, then refreshing or sorting, does not throw a FlutterError.
• Existing default filter behaviour is unchanged.