Gradient marker colors are removed when undoing marker movement in the Inspector Gradient field

Steps to reproduce:

1. Create a new 3D URP project
2. Drag the attached "Gradient.cs" script into the Project window
3. In the Hierarchy, create a new GameObject and drag the script onto it
4. Select the created GameObject
5. In the Inspector, click on the Gradient field
6. Click on any of the bottom markers
7. Change its color
8. Move the marker
9. Click Ctrl+Z to undo
10. Observe the behavior

Actual results: The selected color is removed from the marker and marker returns to initial position. Undo removes all marker changes, not just the selected one

Expected results: Only the last action should be undone. The marker returns to its initial position,  while keeping the previously assigned color

Reproducible with versions: 6000.0.75f1, 6000.3.15f1, 6000.4.7f1, 6000.5.0b8, 6000.6.0a4

Tested on (OS): Ubuntu 24.04, macOS 26.4

Notes:

• Undo removes all marker changes, not just the one selected
• On macOS if a gradient preset is selected and the marker is moved, undo removes the entire gradient and it becomes white
• Before [Internal link], undo may keep the marker color but fail to restore the marker position correctly