Preserve extension dtypes in groupby reductions and exclude grouping-key columns by identity

[galipremsagar]

Summary

Split out from #22289. Two related fixes that together restore pandas-3 behavior for groupby reductions on extension-typed value columns:

Output dtype for int-returning aggregations on StringDtype

• COUNT/SIZE/ARGMIN/ARGMAX now return np.int64 (matching pandas 3) instead of Int64Dtype/int64[pyarrow].
• NUNIQUE always casts to np.int64.
• Series.groupby.size() on string[pyarrow] with na_value=pd.NA now returns Int64Dtype to match pandas 3's specific behavior for that storage/na_value combination.

Identity-based exclusion of grouping-key columns

Pandas excludes a value column whose underlying object is the same as the grouping Series' column (i.e., df.groupby(df["a"]) drops "a" from the aggregated values). This was missing in cuDF.

A new _collect_series_key_column_names helper captures this identity information before nans_to_nulls() breaks it under mode.pandas_compatible, and threads matched column names through _Grouping to populate _named_columns. The check is restricted to DataFrame inputs so that Series.groupby(self) (used internally by Series.value_counts, Series.mode, etc.) doesn't falsely match the Series against itself and empty the aggregation result.

Tests

python/cudf/cudf/tests/groupby/test_reductions.py:

• test_groupby_string_int_returning_aggs_dtype covers count/nunique/size across the four StringDtype storage/na_value combinations.
• test_groupby_series_identity_column_exclusion and test_groupby_series_copy_no_column_exclusion exercise the matched/non-matched paths.
• test_groupby_series_self_does_not_exclude guards against the regression where Series.groupby(self) empties the aggregation.

Conftest

Removes 75 NODEIDS_THAT_FAIL entries that now pass on the regular path:

• test_string_dtype_all_na[*-{count,size,nunique}-*] (60 entries) — fixed by the int-returning dtype change.
• 15 nunique- and identity-related entries: test_size_strings[string=string[pyarrow]], test_groupby_column_index_in_references, test_groupby_nonstring_columns, test_groupby_series_with_name, several test_nunique_* and test_duplicate_columns[nunique-*], etc.

Relationship to #22289

This is one of the four split PRs requested in the review on #22289. #22289 retains only the get_dtype_of_same_kind change; the remaining three split PRs are #string-sum / #bool-any-all / #min-count.

Some test_string_dtype_all_na[*-{sum,all,any,min,max,first,last}-*] parametrizations exercise both this PR's grouping-key-exclusion logic and another split PR's reduction logic, so the corresponding xfail entries stay until both halves merge.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[galipremsagar]

/okay to test 3e7e9af