Closes #5335: Bug: pd.array fails on Arkouda-backed Categorical with dtype specified

[ajpotts]

Summary

Fixes an issue where calling pd.array() on an Arkouda-backed
Categorical with an explicit dtype would fail with a
NotImplementedError.

The failure occurred because pd.array() routed through
ArkoudaArray._from_sequence, which ultimately attempted to iterate
over the Categorical. Arkouda intentionally disallows iteration on
Categorical objects to prevent implicit data transfer from the server.

This change introduces a safe conversion path that: - Detects
Arkouda-backed Categorical inputs in _from_sequence - Extracts the
server-side categorical codes - Casts the codes if a dtype is
provided - Constructs the ArkoudaArray directly from the server-side
pdarray

This avoids iteration entirely and preserves server-side semantics.

---

Root Cause

pd.array(cat, dtype="ak_int64") triggered:

1. ArkoudaArray._from_sequence
2. ak_array(scalars, ...)
3. list(scalars) inside ak_array
4. Categorical.__iter__, which raises NotImplementedError

Since iteration is intentionally blocked for Categoricals, a direct
server-side conversion path was required.

---

Changes

_arkouda_array.py

• Added special-case handling for Arkouda Categorical in
  _from_sequence
• Extract categorical codes directly
• Cast codes when a dtype is provided
• Return cls(codes) without invoking ak_array

Tests

Added:

def test_pd_array_with_dtype_on_ak_categorical_should_not_iterate(self):

This verifies that:

• pd.array(cat, dtype="ak_int64") succeeds
• The resulting values match cat.codes
• No iteration occurs

---

Behavioral Impact

Before: - pd.array(Categorical(...), dtype=...) raised
NotImplementedError

After: - Returns an ArkoudaArray backed by categorical codes - No
implicit data transfer - Fully server-side conversion path

---

Example

import arkouda as ak
import pandas as pd
from arkouda.pandas import Categorical

cat = Categorical(ak.array(["a", "a", "b"]))
arr = pd.array(cat, dtype="ak_int64")

# arr now contains the categorical codes

Closes #5335: Bug: pd.array fails on Arkouda-backed Categorical with dtype specified