Closes #5303: Pandas ExtensionArray: allow dtype=ak for generic Arkouda-backed arrays

[ajpotts]

Summary

This PR introduces a generic Arkouda pandas dtype, dtype="ak", allowing users to construct Arkouda-backed pandas arrays without specifying a concrete Arkouda dtype (e.g. ak_int64, ak_string) up front.

The new generic dtype improves ergonomics and aligns Arkouda’s pandas integration with standard pandas patterns such as dtype="string" or dtype="category".

Motivation

Prior to this change, users had to explicitly specify a concrete Arkouda dtype when constructing pandas objects:

pd.array(data, dtype="ak_int64")
pd.Series(data, dtype="ak_float64")

This is unnecessarily verbose and diverges from typical pandas usage, where users usually select a backend or family and allow the system to infer the concrete dtype.

With this PR, users can now write:

pd.array(data, dtype="ak")
pd.Series(data, dtype="ak")

and rely on Arkouda to infer the appropriate concrete dtype.

What’s in this PR

1. Generic ArkoudaDtype

A new pandas ExtensionDtype, ArkoudaDtype, is introduced and registered under the name "ak".

Key properties:

• dtype="ak" resolves to ArkoudaDtype
• construct_array_type() returns ArkoudaExtensionArray
• Acts as a dispatcher, not a concrete storage dtype

2. Factory-style dispatch in _from_sequence

ArkoudaExtensionArray._from_sequence has been refactored into a true factory:

• Normalizes pandas-provided dtypes, treating "ak" / ArkoudaDtype as a request for backend inference
• Converts Python / NumPy inputs to Arkouda objects once
• Dispatches based on the resulting Arkouda type:
  • pdarray → ArkoudaArray
  • Strings → ArkoudaStringArray
  • pandas-style Categorical → ArkoudaCategoricalArray

This makes _from_sequence the single construction choke point used by pandas when dtype="ak" is specified.

3. Updated documentation

The _from_sequence docstring was updated to accurately reflect:

• Factory/dispatcher behavior
• Post-conversion dispatch
• The semantics of dtype="ak" vs concrete Arkouda dtypes
• pandas construction context (pd.array(..., dtype="ak"))

4. Comprehensive tests

New tests verify that dtype="ak" correctly dispatches for:

• Numeric data (int64, float64)
• Strings
• Arkouda pandas Categorical
• Both pd.array and pd.Series construction paths

Tests also document the required construction pattern for categoricals (pd.array(..., dtype="ak") prior to Series) to avoid pandas eager iteration.

Non-goals / Follow-ups

• astype("ak") behavior is intentionally out of scope for this PR
• No changes to existing concrete Arkouda dtype strings
• No changes to pandas materialization or iteration semantics

These can be addressed in follow-up PRs if desired.

Impact

• Significantly improves usability of Arkouda’s pandas ExtensionArray API
• Reduces boilerplate for exploratory and backend-agnostic code
• Provides a clean foundation for future dtype-related enhancements

---

Example

import pandas as pd

pd.array([1, 2, 3], dtype="ak")        # ArkoudaArray (int64)
pd.array(["a", "b"], dtype="ak")       # ArkoudaStringArray
pd.Series([1.0, 2.0], dtype="ak")      # ArkoudaArray (float64)

Closes #5303: Pandas ExtensionArray: allow dtype=ak for generic Arkouda-backed arrays

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