[Bug]: PGF file produces incorrect whitespace in the legend when using certain documentclass.

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### Bug summary

When using Matplotlib to generate PGF files with the `IEEEtran` document class, I noticed that if the legend font size is smaller than 5, extra whitespace appears on the right side. Currently, my workaround is to forcibly override the default `article` document class, which avoids this issue.

Would it be possible to provide an interface allowing users to directly modify the documentclass used by Matplotlib? Below is the reproducible code and a screenshot demonstrating the issue.

```latex
\documentclass{IEEEtran}
\usepackage{pgf}
\begin{document}
\input{main.pgf}
\end{document}
```

Overriding the default definition can resolve this issue.

```python
import matplotlib
import matplotlib.pyplot as plt
import matplotlib.backends.backend_pgf
matplotlib.backends.backend_pgf._DOCUMENTCLASS = r"\documentclass{IEEEtran}"
if __name__ == '__main__':
matplotlib.rcParams['pgf.texsystem'] = 'pdflatex'
fig, ax = plt.subplots(figsize=(2, 2),constrained_layout=True)
ax.plot([1, 2, 3, 4, 5], [5, 5, 5, 5, 5], label='legend')
ax.plot([1, 2, 3, 4, 5], [5, 5, 5, 5, 5], label='test legend')
legend=ax.legend(fontsize=2)
pfg_file = 'main.pgf'
plt.savefig(pfg_file, transparent=True)
plt.close()
```

### Code for reproduction

```Python
import matplotlib
import matplotlib.pyplot as plt
if __name__ == '__main__':
matplotlib.rcParams['pgf.texsystem'] = 'pdflatex'
fig, ax = plt.subplots(figsize=(2, 2),constrained_layout=True)
ax.plot([1, 2, 3, 4, 5], [5, 5, 5, 5, 5], label='legend')
ax.plot([1, 2, 3, 4, 5], [5, 5, 5, 5, 5], label='test legend')
legend=ax.legend(fontsize=2)
pfg_file = 'main.pgf'
plt.savefig(pfg_file, transparent=True)
plt.close()
```

### Actual outcome


### Expected outcome


### Additional information

_No response_

### Operating system

_No response_

### Matplotlib Version

3.10.0

### Matplotlib Backend

_No response_

### Python version

3.12.4

### Jupyter version

_No response_

### Installation

pip

Inspect matplotlib.backends.backend_pgf and the _DOCUMENTCLASS override; run the supplied Python/LaTeX reproduction with IEEEtran and a small legend font. Done means the document class can be selected through a supported interface and the reported extra legend whitespace is addressed without the private override.

Written by the indexing model from the issue text.