AttributeError: NumberLine object has no attribute 'default_numbers_to_display' when calling get_number_mobjects()

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## Description of bug / unexpected behavior
When attempting to use `get_number_mobjects()` on a `NumberLine` object, I encountered the following error:

```
AttributeError: NumberLine object has no attribute 'default_numbers_to_display'
```

It seems like the method `get_number_mobjects()` internally calls `self.default_numbers_to_display()`, but this method is either missing or no longer available in the current implementation of `NumberLine`.

## Expected behavior
Calling `get_number_mobjects()` with no arguments should use default numbers (e.g., from `x_range`) to generate label mobjects.

## How to reproduce the issue
1. Create a `NumberLine` object.
2. Call `get_number_mobjects()` without passing any explicit numbers.
3. Run the scene.

Code for reproducing the problem

```py
from manim import *

class IntroductionToComplexNumbers(Scene):
def construct(self):
# Create real number line
real_number_line = NumberLine(
x_range=[-5, 5, 1],
length=10,
include_tip=False,
)
real_number_line_labels = real_number_line.get_number_mobjects()
self.add(real_number_line, *real_number_line_labels)
```

## Logs
Terminal output

[Logs](https://pastebin.com/bW1jRatq)

## System specifications

System Details

```
Environment

* **OS:** Windows 11
* **Manim version:** (e.g., v0.19.0)
* **Python version:** (e.g., 3.10)
* **Installation:** pip
```

Start with the NumberLine implementation and the get_number_mobjects() entry point described in the report. Reproduce the provided scene, then verify that calling the method without arguments no longer raises AttributeError and creates labels from x_range.

Written by the indexing model from the issue text.