Fix Error when calling `.get_number_mobjects()` with no arguments in NumberLine

[irvanalhaq9]

Overview: What does this pull request change?

Fix #4244

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

Calling get_number_mobjects() in NumberLine without any arguments caused error.
This PR resolves this issue (see: #4244).

Changes

• Introduced method get_numbers_to_display():
  • Extracted from the logic previously embedded in add_numbers().
• Updated get_number_mobjects():
  • Uses get_numbers_to_display() when numbers is not provided.
• Simplified add_numbers():
  • Moved the logic for determining which numbers to include and which numbers to exclude to the get_numbers_to_display() method. This allows add_number() to focus only on adding the number objects to the axis.
  • Delegates label creation to get_number_mobjects().
  • Removed redundant checks of font_size and label_constructor, as these are already handled inside get_number_mobject().

Links to added or changed documentation pages

No changes in the documentation: https://manimce--4248.org.readthedocs.build/en/4248/reference/manim.mobject.graphing.number_line.NumberLine.html#manim.mobject.graphing.number_line.NumberLine

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