Fix incorrect assignment in Mobject.put_start_and_end_on

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Overview: What does this pull request change?

PR #3718 changed the behavior of Mobject.put_start_and_end_on when start == end, however the assigment to self.points was incorrect. The existing code assigned the Point3D directly to self.points, that then becomes an array with shape (3,), while instead it should really have shape (1, 3).

This commit fixes the incorrect code and associated test.

Motivation and Explanation: Why and how do your changes improve the library?

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

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Rebased code to resolve conflict with #4027

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Explicitly mentioning the author and reviewer of #4027. @chopan050 @JasonGrace2282 what do you think of this fix?

[chopan050]

what do you think of this fix?

IMO, it seems like the best course of action, given that this is a Mobject method.

My only gripe with this is that, if you try to call this method on a closed VMobject such as Circle or Square, the VMobject ends up having a single point in points, which I consider hacky and potentially undesired. Cairo VMobjects should have 4N points and OpenGLVMobjects should have 3N points... except in the case where there's a single lingering point indicating the start of a potentially new path, which, IMO, is handled weirdly.

I've been raising some discussions in Discord where I propose rewriting the points of VMobject (and Mobject in general), but it will have to wait until the experimental branch is finished, since this is a huge change.

Meanwhile, IMO, it makes sense to override VMobject.put_start_and_end_on() such that it becomes a straight line between start and end which is properly defined by 4 points (3 in OpenGL), but I'd like to ask the rest of the developers about it:

class VMobject(Mobject):
    # ...
    def put_start_and_end_on(self, start: Point3DLike, end: Point3DLike) -> self:
        curr_start, curr_end = self.get_start_and_end()
        if np.all(curr_end - curr_start == 0):
            self.clear_points()
            self.start_new_path(start).add_line_to(end)
            return self
        return super().put_start_and_end_on(start, end)