Closes #5287: alignment tests for arkouda.numpy.strings

[ajpotts]

Add NumPy alignment tests for Arkouda Strings

Summary

This PR introduces a comprehensive NumPy-alignment test suite for ak.Strings
and wires it into the existing NumPy test runs. The goal is to explicitly
document, enforce, and track parity (and known divergence) between Arkouda
Strings and NumPy/Python string semantics.

What’s included

• New test file:
  • tests/numpy/alignment_verification/strings_alignment.py
• Added to pytest.ini so it runs with the rest of the NumPy alignment suite
• Coverage for:
  • Case operations (lower, upper, title, capitalize)
  • Predicate methods (isdigit, isalpha, isalnum, etc.)
  • Binary comparisons (==, !=, scalar comparisons)
  • Indexing and slicing
  • Regex-based operations (contains)
  • Error semantics (size mismatch, invalid regex)

Handling known divergences

Several behaviors currently differ from NumPy/Python semantics. These are
intentionally captured using xfail(strict=True) so that:

• CI stays green today
• Tests automatically fail once Arkouda semantics are updated

Tracked divergences:

• Unicode case folding
  • Example: Greek capital sigma does not lowercase to \u03C3
  • Tracked in Issue Unicode case-folding mismatch between Arkouda Strings and NumPy #5288
• Strings.istitle() semantics
  • Returns True for digit-only and whitespace-only strings
  • Tracked in Issue Strings.istitle semantics do not match Python / NumPy #5289
• Strings.isnumeric() semantics
  • Returns True for the empty string
  • Tracked in Issue Strings.isnumeric returns True for empty string (differs from Python / NumPy) #5290

Why this matters

• Makes Arkouda’s string semantics explicit and testable
• Aligns with the existing NumPy alignment philosophy used for pdarray
• Provides a clear roadmap for future semantic fixes without losing signal

Reviewer notes

• xfail(strict=True) is used deliberately to ensure future fixes are detected
• Unicode characters in reasons are escaped to satisfy Ruff (RUF001)
• Regex error tests expect re.error / PatternError (Python 3.13+ behavior)

Closes #5287: alignment tests for arkouda.numpy.strings

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