DecisionTreeRegressor with absolute error criterion: non-optimal split

### Describe the bug

While working on fixing the issue https://github.com/scikit-learn/scikit-learn/issues/9626, I noticed that in some cases, the current implementation of `DecisionTreeRegressor(criterion="absolute_error")` doesn't not find the optimal split in some cases, when sample weights are given.

It seems to only happen with a small number of points, and the chosen split is not too far from the optimal split.

My PR for https://github.com/scikit-learn/scikit-learn/issues/9626 will fix this one too. I'm openning this issue only to document the current behavior.

### Steps/Code to Reproduce

```python
import numpy as np
from sklearn.tree import DecisionTreeRegressor

def abs_error_of_a_leaf(y, w):
    return min((np.abs(y - yi) * w).sum() for yi in y)

def abs_error_of_leaves(leaves, y, w):
    return sum(abs_error_of_a_leaf(y[leaves == i], w[leaves == i]) for i in np.unique(leaves))

X = np.array([[1], [2], [3], [4], [5]])
y = np.array([1, 1, 3, 1, 2])
w = np.array([3., 3., 2., 1., 2.])

reg = DecisionTreeRegressor(max_depth=1, criterion='absolute_error')
sk_leaves = reg.fit(X, y, sample_weight=w).apply(X)
print("leaves:", sk_leaves, "total abs error:", abs_error_of_leaves(sk_leaves, y, w))
# prints [1 1 1 1 2] and 4.0
# If you look at the values of X, y, w, it's easy enough to doubt this split is the best

expected_leaves = np.array([1, 1, 2, 2, 2])
print("total abs error:", abs_error_of_leaves(expected_leaves, y, w))
# prints 3.0 => indeed, the split returned by sklearn is not the best
```

### Expected Results

Chooses a split that minimizes the AE.

### Actual Results

Prints:
```
leaves: [1 1 1 1 2] total abs error: 4.0
total abs error: 3.0
```

Showing the chosen split is not optimal.

### Versions

```shell
System:
    python: 3.12.11 (main, Aug 18 2025, 19:19:11) [Clang 20.1.4 ]
executable: /home/arthur/dev-perso/fast-mae-split/.venv/bin/python
   machine: Linux-6.14.0-29-generic-x86_64-with-glibc2.39

Python dependencies:
      sklearn: 1.7.1
          pip: None
   setuptools: 80.9.0
        numpy: 2.2.6
        scipy: 1.16.1
       Cython: None
       pandas: None
   matplotlib: 3.10.6
       joblib: 1.5.2
threadpoolctl: 3.6.0

Built with OpenMP: True

threadpoolctl info:
       user_api: blas
   internal_api: openblas
    num_threads: 16
         prefix: libscipy_openblas
       filepath: /home/arthur/dev-perso/fast-mae-split/.venv/lib/python3.12/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so
        version: 0.3.29
threading_layer: pthreads
   architecture: Haswell

       user_api: blas
   internal_api: openblas
    num_threads: 16
         prefix: libscipy_openblas
       filepath: /home/arthur/dev-perso/fast-mae-split/.venv/lib/python3.12/site-packages/scipy.libs/libscipy_openblas-68440149.so
        version: 0.3.28
threading_layer: pthreads
   architecture: Haswell

       user_api: openmp
   internal_api: openmp
    num_threads: 16
         prefix: libgomp
       filepath: /home/arthur/dev-perso/fast-mae-split/.venv/lib/python3.12/site-packages/scikit_learn.libs/libgomp-a34b3233.so.1.0.0
        version: None
```

COMMENT:
What do you mean by "sample issues in Decision trees"?

In my PR https://github.com/scikit-learn/scikit-learn/pull/32100, I fixed this bug by only changing how `MAE(RegressionCriterion)` was implemented. I can't tell what was wrong with the previous one though.

I added the example from this issue as a test case btw.

And btw, if you're familiar with the tree module and you're looking to review a big PR, feel free to review my PR 😜

Labels: Bug