Add snap-to-point case where first point is snapped

[archibald1418]

Motivation and context

Found a bug in snap-to-point #10421 whereby a snapped polygon's first point is not snapping when it is on an another shape.
The following points snap correctly.
This bug is also crucial for 'Join polygons' #10362 feature, since in order for polygons to join correctly, they must have shared sides, with no gaps in between, which is typically achieved with the help of snap-to-point feature (if you are adding a polygon on top of something else)

Screen.Recording.2026-04-16.at.20.22.27.mov

How has this been tested?

Added a test scenario:

• draw a rectangle
• turn on snap-to-point
• draw a polygon, starting from the rectangle's first point. Click in the region of the two point (previous test proved that delta region of 1.3*r is correct), put third point away, resulting in a triangle. Thus, the both shapes have a common side, the triangle should visually sit perfectly on the rectangle
• check that the polygon's and the rectangle's first point are identical in global coordinates (=snapping is correct)

Currently the test is being skipped, awaiting for the fix

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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