`DecisionBoundaryDisplay.from_estimator` only displays up to 7 distinct colours

### Describe the bug

I was trying to use `DecisionBoundaryDisplay.from_estimator` to display different regions classified by `NuSVC`:

<img width="455" height="355" alt="Image" src="https://github.com/user-attachments/assets/ffd7d137-b7f1-46a9-bd63-b967903617ce" />

My expectation was to find 11, but by visually counting, I was only able to find 10.
Also, some colours were obviously duplicated in this figure.
This is how it actually should look like:

<img width="455" height="355" alt="Image" src="https://github.com/user-attachments/assets/29b0c239-1a84-47fd-96c1-57d72f0d02c8" />

## Proposed fix

I guess that this will be fixed at some point by https://github.com/matplotlib/matplotlib/commit/0eadaf0effeb424e2b385693fbf6ac8a21da4d84. However, this fix is not yet available in the stable versions of matplotlib.
Manually changing the "default"/hard-wired value [here](https://github.com/matplotlib/matplotlib/blob/v3.10.8/lib/matplotlib/contour.py#L1020) fixes the problem.
The best fix, thus, is probably for scikit-learn to specify the `levels` parameter on its own here:
https://github.com/scikit-learn/scikit-learn/blob/bd0dd87f792ee88c8ec70248299aae779b51ac16/sklearn/inspection/_plot/decision_boundary.py#L220
That line should look something like this:
```py
self.surface_ = plot_func(self.xx0, self.xx1, self.response, <levels>, **kwargs)
```
where `<levels>` is replaced by the appropriate amount of levels.

The case when `self.response.ndim == 3` may also be affected, but I have not yet tried out that one

### Steps/Code to Reproduce

```py
import numpy as np
from sklearn.inspection import DecisionBoundaryDisplay
from sklearn.svm import NuSVC
import matplotlib.pyplot as plt

# List of some 11 distinct points
pts = np.array([[-1, -1], [-2, -1], [1, 1], [2, 1],
                [2, 2], [3, 2], [3, 3], [4, 3],
                [4, 4], [5, 4], [5, 5]])
svc = NuSVC(nu=0.1)
svc.fit(pts, [1,2,3,4,5,6,7,8,9,10,11])
DecisionBoundaryDisplay.from_estimator(
    svc,
    pts,
    response_method="predict",
    cmap="gist_rainbow"
)
plt.show()
```

### Expected Results

<img width="455" height="355" alt="Image" src="https://github.com/user-attachments/assets/adbec1fc-5341-4842-a314-98d2d25f3dcd" />

### Actual Results

<img width="455" height="355" alt="Image" src="https://github.com/user-attachments/assets/54b39c7d-c6e8-4e83-ba04-cfb5daf95459" />

### Versions

```shell
System:
    python: 3.12.7 | packaged by Anaconda, Inc. | (main, Oct  4 2024, 13:27:36) [GCC 11.2.0]
executable: /home/nico/miniconda3/envs/pca-assessment/bin/python
   machine: Linux-6.14.0-36-generic-x86_64-with-glibc2.39

Python dependencies:
      sklearn: 1.7.2
          pip: 24.3.1
   setuptools: 72.1.0
        numpy: 2.3.3
        scipy: 1.16.2
       Cython: None
       pandas: 2.3.2
   matplotlib: 3.10.6
       joblib: 1.4.2
threadpoolctl: 3.5.0

Built with OpenMP: True

threadpoolctl info:
       user_api: blas
   internal_api: openblas
    num_threads: 16
         prefix: libscipy_openblas
       filepath: /home/nico/miniconda3/envs/pca-assessment/lib/python3.12/site-packages/numpy.libs/libscipy_openblas64_-8fb3d286.so
        version: 0.3.30
threading_layer: pthreads
   architecture: Haswell

       user_api: blas
   internal_api: mkl
    num_threads: 12
         prefix: libmkl_rt
       filepath: /home/nico/miniconda3/envs/pca-assessment/lib/libmkl_rt.so.2
        version: 2025.0-Product
threading_layer: gnu

       user_api: openmp
   internal_api: openmp
    num_threads: 16
         prefix: libgomp
       filepath: /home/nico/miniconda3/envs/pca-assessment/lib/libgomp.so.1.0.0
        version: None
```

COMMENT:
I confirmed the issue and proposed a fix in the linked PR: https://github.com/scikit-learn/scikit-learn/pull/32867 

Decision boundary plot before:

<img width="568" height="430" alt="Image" src="https://github.com/user-attachments/assets/04bac9b4-2572-498e-acbf-f0d4f8ff9c14" />

And after:

<img width="567" height="431" alt="Image" src="https://github.com/user-attachments/assets/ba017d03-72a3-4601-8cd0-dd295b107f81" />

Labels: Bug