add an object selector to skrub.selectors

[omkar-334]

Description

Closes #2124.

Adds skrub.selectors.object(), a selector that picks columns whose dtype is object (pandas) or pl.Object (polars).

The selector is a thin wrapper around the existing sbd.is_object dispatch (skrub/_dataframe/_common.py:868).

Checklist

• I have read the contributing guidelines
• I have added tests that verify the bug fix
• I have added an entry to CHANGES.rst describing the fix
• My code follows the code style of this project
• I have checked my code and corrected any misspellings

How Has This Been Tested?

Test script

from skrub import selectors as s import pandas as pd import polars as pl

def demo_pandas():
print("=== pandas frame ===")
df = pd.DataFrame(
{
"mixed": pd.Series(["a", 1]),
"numeric": [1, 2],
"ext_string": pd.Series(["a", "b"]).convert_dtypes(),
}
)
print("dtypes:")
print(df.dtypes.to_string())
selected = s.select(df, s.object()).columns.tolist()
print("s.object() selected:", selected)
print()

def demo_polars():
print("=== polars frame ===")
df = pl.DataFrame(
{
"obj": pl.Series([object(), object()], dtype=pl.Object),
"str": ["a", "b"],
"num": [1, 2],
}
)
print("schema:", df.schema)
selected = s.select(df, s.object()).columns
print("s.object() selected:", selected)
print()

demo_pandas()
demo_polars()

AI Disclosure

• This PR contains AI-generated code
  • I have tested the code generated in my PR
  • I have read and understood every line that has been generated by the AI agent
  • I can explain what the AI-generated code does