TEST: Focus MultiIndex sort_index coverage

[vyasr]

Description

Replace the broad MultiIndex sort_index Cartesian matrix with focused cases that separate sort semantics from lifecycle behavior.

Coverage accounting

• The selected sort cases retain every previous level spelling: numeric positions, level names, single-item lists, multi-level lists, None, and the invalid level selection.
• They exercise ascending and descending order and both null placements where those choices contribute distinct behavior.
• A dedicated lifecycle matrix retains every inplace and ignore_index combination, and a separate case retains the axis="index" alias.

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