TEST: Split str.cat coverage by behavior

[vyasr]

Description

Replace the broad str.cat Cartesian product with behavior-focused cases and isolate index alignment from unrelated argument combinations. The change removes repetition, not input forms or observable behaviors.

Coverage accounting

• Join cases retain no separator, empty and multi-character separators, null inputs, and na_rep handling.
• Elementwise cases retain list, tuple, Series, Index, ndarray, mixed-sequence, and heterogeneous-sequence others forms.
• Series alignment cases retain reindexing, matching and partially overlapping integer/string indices, including the existing expected failures.
• The existing 72 others=Index cases remain separate and unchanged.

Checklist

• I am familiar with the CONTRIBUTING.md guidelines.
• New or existing tests cover these test-only changes.
• Documentation updates are not needed.

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[vyasr]

/ok to test

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📝 Summary

Summary by CodeRabbit

• Tests
  • Expanded string concatenation test coverage for joins, null handling, and supported input types, including lists, tuples, Series, Indexes, NumPy arrays, and mixed collections.
  • Added coverage for Series alignment and index-based inputs.
  • Consolidated related scenarios into shared test coverage for improved consistency and clarity.
  • Updated expected index comparisons to respect the source Series index type.

Walkthrough

Changes

The .str.cat tests now use a shared pandas/cuDF comparison helper. Coverage includes scalar joins, supported input forms, null handling, NumPy inputs, index-based inputs, and Series index alignment.

String concatenation test coverage

Layer / File(s)	Summary
Comparison helper and core concatenation cases python/cudf/cudf/tests/series/accessors/test_str.py	Adds shared result comparison and focused tests for joins, elementwise concatenation, null values, separators, replacements, Series, and Index inputs.
Supported input forms python/cudf/cudf/tests/series/accessors/test_str.py	Adds coverage for lists, tuples, Series, Indexes, NumPy arrays, nested collections, and heterogeneous inputs.
Series index alignment python/cudf/cudf/tests/series/accessors/test_str.py	Adds alignment cases for reindexing, default and partial indexes, matching and disjoint string indexes, separators, null replacements, and expected unsupported string-index cases.

Priority: ⬇️ Low — Defer this test-only refactor because it reorganizes existing `str.cat` coverage without changing public behavior.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🔵 Low · up to 00e94

The test refactor remains mergeable with owner awareness, but array-input coverage should retain CuPy and Numba cases to avoid reduced validation of supported inputs.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: splitting str.cat test coverage by behavior.
Description check	✅ Passed	The description directly explains the test restructuring, retained coverage, and separation of index alignment cases.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
In `@python/cudf/cudf/tests/series/accessors/test_str.py`:
- Around line 2775-2780: Add CuPy-array and Numba-array variants to the
input-form parameter matrix for the .str.cat tests, alongside the existing
tuple-of-ndarrays case. Ensure both cases exercise conversion through
__cuda_array_interface__ while preserving the expected concatenated results.
- Line 2708: Update the expected-index normalization in the StringMethods.cat
test so it uses the fixed input expectation, such as ps.index.dtype, rather than
got.index.dtype. Keep expect independent of the result under test so incorrect
index dtypes for nested NumPy others remain detectable.

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