Closes #5155: Bug: MultiIndex .lookup() attempts illegal dtype cast for tuple keys

[ajpotts]

PR Description: Improve Index.lookup and MultiIndex.lookup semantics

This pull request refines type-handling, error messaging, and row/column matching
behavior for both Index.contains and MultiIndex.lookup.
It also adds a regression test ensuring that mixed-dtype tuple keys do not
trigger incorrect scalar casting.
The changes improve correctness, readability, and alignment with Pandas semantics.

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Summary of Changes

1. Index.lookup

File: arkouda/pandas/index.py

• Updated the return-value docstring to correctly indicate that the result is a
  boolean pdarray of length len(self).
• Improved the TypeError description to reflect that the function expects a
  value convertible into an Arkouda array.
• Removed an unused import (akint64).

Motivation:
Clarifies semantics and avoids stale / unused imports.

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2. MultiIndex.lookup

File: arkouda/pandas/index.py

Major improvements to validation, dtype behavior, and membership logic:

Validation

• Rejects keys that are not list or tuple.
• Enforces that the key length matches nlevels with a clear ValueError.

Two explicit code paths

1. Per-level arkouda arrays (e.g. list of pdarray / Strings)
   Delegated directly to in1d(self.index, key) for vectorized matching.

2. Scalar tuple keys (e.g., (1, "red"))

   • Scalars are wrapped into length‑1 Arkouda arrays without casting dtypes.
   • Prevents accidental coercion of string scalars into numeric types.

This behavior aligns better with Pandas and eliminates subtle dtype bugs.

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3. New Test: Mixed-dtype tuple lookup

File: tests/pandas/index_test.py

Added test test_multiindex_lookup_tuple_mixed_dtypes:

• Ensures that a scalar mixed-type key like (1, "red"):
  • Does not cast "red" into numeric types.
  • Produces correct row-level matching.
• Verifies the mask is [True, False, False, False] for the provided example.

Motivation:
Prevents regressions and captures a real-world bug scenario.

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Why This Matters

• Fixes subtle multi-dtype matching bugs in MultiIndex.lookup.
• Moves Arkouda’s Pandas-backed behavior closer to Pandas semantics.
• Improves test coverage around a previously fragile API surface.
• Supports downstream work on joins, grouping, and Index alignment.

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Backward Compatibility

• No breaking API changes.
• Behavior is now more correct for mixed-type keys and aligns with expected user intuition.

Closes #5155: Bug: MultiIndex .lookup() attempts illegal dtype cast for tuple keys