BUG: Fix return type of loc/iloc

[sanggon6107]

• [X] closes #60600
• [X] Tests added and passed if fixing a bug or adding a new feature
• [X] All code checks passed.
• [ ] Added type annotations to new arguments/methods/functions.
• [X] Added an entry in the latest doc/source/whatsnew/v3.0.0.rst file if fixing a bug or adding a new feature.

Description of Linked Issue

loc/iloc inconsistently returns dtype. For example,

>>> import pandas as pd
>>> df = pd.DataFrame([['a', 1., 2.], ['b', 3., 4.]])

>>> df.loc[0, [1, 2]]
1    1.0
2    2.0
Name: 0, dtype: object

>>> df[[1, 2]].loc[0]
1    1.0
2    2.0
Name: 0, dtype: float64

>>> df.loc[[0, 1], 1]
0    1.0
1    3.0
Name: 1, dtype: float64

This behaviour seems to happen following the below sequence:

1. For axis=0, BlockManager.fast_xs() returns a cross-section of df, determining the dtype as object, since df.loc[0,:] is supposed to include 'a'.
2. For axis=1, NDFrame._reindex_with_indexers() returns the result, not additionally inferring the dtype of the result.

Proposed Solution

Based on the above examples, we can conclude that this issue only apprears where axis[0]=int amd axis[1]=list/slice - loc[int/slice]. Therefore, I'd like to propose to add the below codes to additionally infer the dtype after the column selection.

    @final
    def _getitem_lowerdim(self, tup: tuple):

...

                # This is an elided recursive call to iloc/loc
                out = getattr(section, self.name)[new_key]
                # Re-interpret dtype of out.values for loc/iloc[int, list/slice].
                # GH60600
                if (
                    i == 0
                    and isinstance(key, int)
                    and isinstance(new_key, (list, slice))
                ):
                    out = out.infer_objects()
                return out

Thanks!