Fixes StartedEditingItem not firing in DataGridEditMode.Cell. The callback only fired in the form/dialog edit modes, so edit-start workflows such as item snapshotting were unreliable when editing cells:

• Raise StartedEditingItem when cell editing begins, both on a row click (EditTrigger.OnRowClick) and when a cell input is edited directly. It is raised once per item and again when editing moves to a different item, using the grid's Comparer for item identity (consistent with selection).
• Keep cell editing clone-free. In cell mode SetEditingItemAsync no longer creates a working-copy clone or opens the edit form; it raises the callback with the source item, which is what cell mode edits in place.
• Form/dialog edit modes are unchanged.

Cell editing has never cloned the row — it writes changes straight to the source item. Routing the cell path through the normal clone (SystemTextJsonDeepCloneStrategy) throws for item types System.Text.Json cannot round-trip (for example a constructor parameter that does not bind back to a property), which breaks cell editing outright (DataGridEditComplexPropertyExpression). Raising the callback with the live source item keeps the cell path independent of the clone strategy.

<MudDataGrid T="Item" Items="@items" ReadOnly="false"
             EditMode="DataGridEditMode.Cell"
             EditTrigger="DataGridEditTrigger.OnRowClick"
             StartedEditingItem="OnStartedEditingItem">
    <Columns>
        <PropertyColumn Property="x => x.Name" />
    </Columns>
</MudDataGrid>

Tests

• StartedEditingItem fires on a direct cell edit, on a row click in cell mode, and via SetEditingItemAsync in cell mode; the edit form stays closed and no working copy is created.
• It is raised once per item, and again when a different item is edited; the dedup honors a custom Comparer.
• It fires for a non-cloneable item type without throwing.
• The existing DataGridEventCallbacks test now also asserts the callback during cell editing.

Behavior change

Calling the public SetEditingItemAsync (or CellContext.Actions.StartEditingItemAsync from a custom template) while EditMode is Cell previously opened the form-style edit dialog. It now raises StartedEditingItem and opens nothing, consistent with cell editing writing directly to the source item.