Closes #5230: ArkoudaExtensionArray arithmetic

[ajpotts]

Enable pandas arithmetic dispatch for Arkouda ExtensionArray

Summary

This PR implements the pandas ExtensionArray arithmetic hook (_arith_method) for
ArkoudaExtensionArray, enabling elementwise arithmetic operations (e.g. +, -, *)
between Arkouda-backed arrays and with scalars.

---

Motivation

Pandas does not automatically dispatch Python operators (__add__, etc.) for
ExtensionArrays. Instead, arithmetic is routed through _arith_method. Without this
hook, expressions like:

pd.array([1, 2, 3], dtype="ak_int64") + pd.array([4, 5, 6], dtype="ak_int64")

raise TypeError.

Implementing _arith_method is required for:

• correct pandas operator dispatch
• future Series / DataFrame arithmetic
• consistency with pandas ExtensionArray contracts

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What’s in this PR

Core functionality

• Adds _arith_method to ArkoudaExtensionArray
  • Supports EA–EA and EA–scalar operations
  • Returns NotImplemented for unsupported operand types
  • Preserves the concrete EA type on return
• Adds _from_data constructor helper for mypy-safe instance creation
• Annotates internal _data attribute for static typing

Typing & correctness

• Uses typing_extensions.Self for precise self-type returns
• Uses NotImplementedType (not the value) in return annotations

Tests

• Adds unit tests covering:
  • EA–EA arithmetic
  • EA–scalar arithmetic
  • NotImplemented propagation
  • User-visible TypeError behavior for unsupported operands
• Existing argsort / NaN placement tests remain unchanged and passing

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Design notes

• Index alignment is intentionally not handled here; pandas performs alignment
  before calling into the ExtensionArray.
• Type coercion and promotion semantics are delegated to the underlying Arkouda
  operations.
• The implementation follows pandas’ recommended EA patterns rather than Python
  operator overloading.

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Example

import pandas as pd

x = pd.array([1, 2, 3], dtype="ak_int64")
y = pd.array([10, 20, 30], dtype="ak_int64")

x + y
# ArkoudaArray([11 22 33])

---

Reviewer notes

• The _from_data helper is intentionally minimal and centralizes EA construction.
• Duck-typing (hasattr(other, "_data")) is used instead of concrete EA imports to
  avoid circular dependencies.
• All changes are localized to the ExtensionArray layer; no pandas behavior is
  modified.

Closes #5230: ArkoudaExtensionArray arithmetic