[Bug] DrawAreaTask saves duplicate points and allows degenerate polygons

### Description
During a refactor of the `DrawAreaTask` state management to adhere to Unidirectional Data Flow (UDF) principles (https://github.com/google/ground-android/pull/3678), several issues were identified in the polygon drawing logic that could lead to invalid geometry being saved and a confusing user experience.

### Detected Issues

1. **Duplicate Points in Storage:** Incomplete polygons were saving duplicate points (specifically duplicating the last vertex), resulting in 4 points instead of 3 for a triangle.
1. **Incorrect Undo Behavior:** The Undo action would remove a committed vertex even when there was a tentative vertex actively being drawn on the screen.
1. **Missing Validation for Degenerate Shapes:** The proximity check (`isTooClose`) failed to trigger when closing a triangle with only 2 committed points, allowing degenerate lines to be formed.
1. **Premature Button Visibility:** The `COMPLETE` button was displayed for closed shapes that didn't have enough distinct points to form a valid polygon (requires 3 distinct points).

### Proposed Fix
The implementation has been refactored in the current branch to:

- Separate committed vertices from the transient tentative vertex in the ViewModel state.
- Clear the tentative vertex upon commit.
- Enforce a minimum of 3 distinct points for closure validation.
- Update proximity checks to cover all edge cases when closing shapes.
- Introduce `PolygonDrawingSession` class to encapsulate list of committed vertices, transient tentative vertex, redo stack, operations like adding a vertex, undoing, and redoing.

### Steps to Reproduce

1. Start drawing an area task.
2. Place 3 points.
3. Observe that the saved incomplete geometry contains 4 points instead of 3.