Closes #5336: ArkoudaExtensionArray.value_counts

[ajpotts]

Summary

This PR adds value_counts implementations for Arkouda-backed pandas ExtensionArrays, including:

• ArkoudaArray
• ArkoudaStringArray
• ArkoudaCategoricalArray

The goal is to provide pandas-compatible behavior for counting unique values while leveraging Arkouda’s server-side aggregation.

Motivation

value_counts is a core pandas API that users expect to work on ExtensionArrays. Prior to this change:

• value_counts was not implemented (or explicitly disabled) for Arkouda-backed arrays
• Users had to materialize data back to NumPy/pandas to compute counts

This PR enables efficient server-side counting and returns a pandas Series with Arkouda-backed index and values where appropriate.

What’s included

ArkoudaArray

• Implements value_counts(dropna=True)
• Supports dropna=True for float64 by filtering NaN values
• Returns an empty Series for empty input
• Notes pandas options not yet implemented (sort, normalize, bins)

ArkoudaStringArray

• Implements value_counts(dropna=True)
• Uses server-side GroupBy.size() on Strings
• Accepts dropna for pandas compatibility (currently inert)

ArkoudaCategoricalArray

• Implements value_counts(dropna=True|False)
• Uses categorical codes + categories for efficient counting
• Respects the categorical na_value when dropna=True

Tests

• Adds unit tests for all three array types
• Verifies correctness against pandas (order-insensitive)
• Covers empty inputs, basic cases, and dropna behavior
• Includes regression tests for common implementation pitfalls

Non-goals / future work

• Pandas options such as sort, ascending, normalize, and bins are not yet implemented
• Result ordering is not guaranteed and tests avoid relying on it

Closes #5336: ArkoudaExtensionArray.value_counts

[codecov]

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