Closes #5404: ArkoudaExtensionArray._cmp_method

[ajpotts]

Summary

This PR implements the pandas ExtensionArray comparison protocol for
ArkoudaArray by adding a fully-typed _cmp_method implementation and
expanding comparison test coverage.

This resolves issues where operations such as:

x = pd.array([1, 3, 4, 1], dtype="ak_int64")
x > 2

would fail due to missing or incorrectly scoped helper methods.

---

Changes

1. Implement _cmp_method

• Added _cmp_method to conform to pandas ExtensionArray API.
• Properly typed with full signature annotations.
• Returns ArkoudaArray | NotImplementedType.
• Delegates to _binary_op to reuse normalization and broadcasting
  logic.
• Fully documented with NumPy-style docstring compliant with
  pydoclint.

2. Harden _coerce_other_for_binop

• Added explicit return type annotation tuple[Any, str].
• Updated docstring to match signature (fixes DOC101, DOC103, DOC203).
• Removed unnecessary comment for clarity.

3. Add Comparison Test Coverage

New tests added to tests/pandas/extension/arkouda_array_extension.py:

• _cmp_method matches dunder operators
• Scalar comparisons
• NumPy array comparisons
• Length mismatch raises ValueError
• Guard test ensuring helper methods exist (prevents indentation
  errors)
• pd.array(..., dtype="ak_int64") > scalar integration test

These tests prevent regression of the original failure mode
(AttributeError: 'ArkoudaArray' object has no attribute '_coerce_other_for_binop').

---

Why This Matters

• Ensures full compatibility with pandas comparison dispatch.
• Prevents silent class-structure regressions (indentation/dedent
  issues).
• Improves test coverage for comparison operations.
• Brings docstrings into full compliance with project linting rules.

---

Validation

• flake8 passes (including pydoclint rules).
• All new comparison tests pass.
• Manual verification of pd.array(..., dtype="ak_int64") > scalar.

---

Closes #5404: ArkoudaExtensionArray._cmp_method

[jaketrookman]

Looks good