Enhance QuickGrid by Exposing Column Metadata for Extensibility

[vendasankarsf3945]

Enhance QuickGrid by Exposing Column Metadata for Extensibility

Description

Exposed the protected Columns property on QuickGrid<TGridItem> to allow derived classes to access the column list.

The _columns list was previously private, preventing developers from creating custom grid components that inherit from QuickGrid and need to enumerate columns (e.g., for CSV export functionality).

Before:

The _columns field in QuickGrid.razor.cs was marked as private, which meant developers building custom grid components that inherit from QuickGrid could not access the column list. This prevented implementing features like CSV export where the component needs to read column headers to build the export file.

For example, when building an exportable grid component that overrides QuickGrid to add CSV export functionality, the derived class had no way to enumerate the defined columns because _columns was private.

After:

The Columns property is now declared as protected, returning an IReadOnlyList<ColumnBase<TGridItem>>. This allows any class that inherits from QuickGrid<TGridItem> to access the list of columns and retrieve properties like Title for each column.

With this change, developers can now create derived classes that implement CSV export by iterating over Columns and reading the Title property of each column to build the header row of the export file.

Fixes #45398

[dotnet-policy-service]

Thanks for your PR, @vendasankarsf3945. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[vendasankarsf3945]

@dotnet-policy-service agree company="Syncfusion, Inc."