`type_of_target` returns 'multiclass' instead of 'continuous' on regression tasks with integer values e.g. house prices.

### Describe the bug

`utils.multiclass.type_of_target` returns 'multiclass' when y is the target of a regression problem that only takes integer values, even when cast to "float".

This appeared when I was working on the Ames housing dataset where prices are integers but represent a regression task. When splitting the dataset into train and test, the number of samples becomes less than half the number of 'classes' i.e. unique values so `type_of_target` raises the warning
"The number of unique classes is greater than 50% of the number "
"of samples. `y` could represent a regression problem, not a "
"classification problem."

This happened while using `RandomForestRegressor` with `oob_score=True` which calls `type_of_target(y)`. It can be fixed on the user side by adding some small non integer float to all elements of the target but I think this should be fixed on the library side. Note that casting the target type to "float" does not fix the issue.

### Steps/Code to Reproduce

```
from sklearn.datasets import fetch_openml
from sklearn.impute import SimpleImputer
from sklearn.model_selection import train_test_split
from sklearn.utils.multiclass import type_of_target

X, y = fetch_openml(data_id=42165, as_frame=True, return_X_y=True)
y = y.astype("float")

X = X.drop(columns="Id")
X = X.select_dtypes("number")

imputer = SimpleImputer(strategy="mean")
X = imputer.fit_transform(X)

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.3)

print(type_of_target(y_test))
```

### Expected Results

Outputs 'continuous' with no warning.

### Actual Results

```
typeoftarget_bug_reproducer.py:17: UserWarning: The number of unique classes is greater than 50% of the number of samples. `y` could represent a regression problem, not a classification problem.
  print(type_of_target(y_test))
multiclass
```
### Versions

```shell
System:
    python: 3.12.3 (main, Feb  4 2025, 14:48:35) [GCC 13.3.0]
executable: /home/gaeta/.sklearn-env/bin/python3
   machine: Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.39

Python dependencies:
      sklearn: 1.8.dev0
          pip: 25.1.1
   setuptools: 80.8.0
        numpy: 2.2.6
        scipy: 1.15.3
       Cython: 3.1.3
       pandas: 2.2.3
   matplotlib: 3.6.0
       joblib: 1.5.2
threadpoolctl: 3.6.0

Built with OpenMP: True

threadpoolctl info:
       user_api: blas
   internal_api: openblas
    num_threads: 14
         prefix: libscipy_openblas
       filepath: /home/gaeta/.sklearn-env/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: 14
         prefix: libscipy_openblas
       filepath: /home/gaeta/.sklearn-env/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: 14
         prefix: libgomp
       filepath: /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0
        version: None
```

COMMENT:
> Isn't it that the time complexity of looking for unique values in a 1D array is O(n)?

According to https://github.com/scikit-learn/scikit-learn/pull/26335#discussion_r1186540871 its O(n*log(n)): 
> np.unique(y) adds another n*log(n) operation. (np.unique sorts the data)


> `check_classification_targets` really feels like the more natural place to put this warning

I agree.

This solves the issue of `RandomForestRegressor` raising the confusing warning, but not the issue of `TargetEncoder`.

I think the issue with `TargetEncoder` is a bit subtle, because it uses the keywords "multiclass"/"binary" and "continuous" to distinguish classification and regression while `type_of_target` uses the same keywords for something else. In the context of `type_of_target`, "multiclass" and "continuous" are not mutually exclusive.

Labels: Documentation