Closes #5105: ArkoudaExtensionArray.__getitem__

[ajpotts]

PR: Unified Indexing & Assignment for Arkouda Extension Arrays

This PR introduces a complete overhaul of __getitem__ and __setitem__ across all Arkouda-backed pandas ExtensionArray types:

• ArkoudaArray
• ArkoudaStringArray
• ArkoudaCategoricalArray

The goal is to bring these classes into full alignment with pandas/NumPy indexing rules, while ensuring all operations are executed entirely on the Arkouda server (no unintended materialization, no to_numpy() or to_ndarray() inside assignment paths).

---

Summary of Changes

⭐ 1. Unified Indexer Normalization

All extension arrays now normalize Python / NumPy indexers the same way:

• int, slice: forwarded directly to the underlying Arkouda object
• NumPy integer arrays → ak.array(..., dtype=int64/uint64)
• NumPy boolean masks → ak.array(..., dtype=bool)
• Python lists of ints/bools → ak.array(...)
• Arkouda pdarray indexers pass through unchanged
• Empty indexers ([], np.array([], ...)) return an empty array of the same type

This ensures consistency across array types and avoids breaking pandas’ internal indexing machinery.

---

⭐ 2. New __getitem__ Implementations

ArkoudaArray

• Mirrors pandas EA semantics exactly.
• Returns a Python scalar for integer indexing and a new ArkoudaArray for everything else.
• Full rewrite with examples and proper Raises section.
• Now fully matches ExtensionArray.__getitem__ type signature.

ArkoudaStringArray

• Correctly distinguishes scalar vs non-scalar returns.
• Ensures doctest examples return object dtype arrays when round-tripped through NumPy.
• Handles all indexer forms without falling through to unsupported Arkouda paths.

ArkoudaCategoricalArray

• Returns a scalar Python string for scalar access and a wrapped categorical EA otherwise.
• Empty indexers produce an empty categorical of type str_.

---

Closes #5105: ArkoudaExtensionArray.getitem

[1RyanK]

Looks good!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@be6b999). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #5106   +/-   ##
========================================
  Coverage        ?   100.00%           
========================================
  Files           ?         4           
  Lines           ?        63           
  Branches        ?         0           
========================================
  Hits            ?        63           
  Misses          ?         0           
  Partials        ?         0           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.