Fix add_points_as_corners not connecting single point to existing path

[irvanalhaq9]

Overview: What does this pull request change?

Fixes #4218.

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

add_points_as_corners() did not include the last point of the existing path, causing no line to be added when only one new point was provided.

This fix explicitly adds the last point of the existing path as the starting corner of the new segment, ensuring the path is continuous and no segment is skipped.

Links to added or changed documentation pages

Docs related to the issue after this fix: https://manimce--4219.org.readthedocs.build/en/4219/examples.html#pointwithtrace

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