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