Add tests for polygon shape, star shape, and grid snapping tools

[tarone-saloni]

Summary

This issue tracks adding #[cfg(test)] test coverage to three currently untested files in the tool and snapping systems. These are good starting points for GSoC test coverage work, as they follow the same pattern already established in ellipse_shape.rs and line_shape.rs.

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Files to Test

1. editor/src/messages/tool/common_functionality/shapes/polygon_shape.rs (~193 LOC)

What to test:

• draw_polygon_simple — drag with ToolType::Shape (polygon mode), assert vertex count and radius
• draw_polygon_square_ratio — drag with ModifierKeys::SHIFT (lock ratio), assert equal dimensions
• draw_polygon_from_center — drag with ModifierKeys::ALT, assert center origin
• draw_polygon_cancel_rmb — use drag_tool_cancel_rmb, assert no layer created
• polygon_decrease_sides — assert decrease_or_increase_sides(true, ...) clamps to minimum 3
• polygon_increase_sides — assert side count increments correctly

Reference pattern: ellipse_shape.rs (same file, same module structure)

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2. editor/src/messages/tool/common_functionality/shapes/star_shape.rs (~172 LOC)

What to test:

• draw_star_simple — drag to create star, assert outer/inner radius values
• draw_star_square_ratio — with ModifierKeys::SHIFT
• draw_star_from_center — with ModifierKeys::ALT
• draw_star_cancel_rmb — assert no layer created on RMB cancel
• Verify inner radius is set to outer_radius / 2.0 by default (Star::update_shape line 161)

Reference pattern: Same as ellipse_shape.rs; use NodeGraphLayer::upstream_node_id_from_protonode with star::IDENTIFIER

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3. editor/src/messages/tool/common_functionality/snapping/grid_snapper.rs (~181 LOC)

What to test:

• rectangular_grid_snap_on_line — point exactly on a grid line snaps with distance ≈ 0
• rectangular_grid_snap_within_tolerance — point within tolerance snaps to nearest line
• rectangular_grid_no_snap_outside_tolerance — point beyond tolerance produces no snap result
• isometric_grid_snap_vertical — snap to vertical lines in isometric grid
• isometric_grid_angle_lines — snap to angled lines (angle_a / angle_b)
• constrained_snap_to_grid_intersection — constrained snap hits grid line correctly

Test Infrastructure

All tests should use the existing EditorTestUtils helper:

#[cfg(test)]
mod test_polygon {
    use crate::test_utils::test_prelude::*;
    // ...

    #[tokio::test]
    async fn draw_polygon_simple() {
        let mut editor = EditorTestUtils::create();
        editor.new_document().await;
        editor.drag_tool(ToolType::Shape, 10., 10., 50., 50., ModifierKeys::empty()).await;
        // assert layer count, vertex count, radius etc.
    }
}