test: cover utils helpers and xsg accessor; fix deprecations

[Divyateja2709]

This PR improves test coverage and reliability by adding unit tests for utility functions and accessor behavior, along with small fixes to ensure correct warnings and robust handling of inputs.

Changes

Tests

• Added tests/test_utils.py:

  • Covers format_bytes
  • Covers asdatetime (None, datetime, cftime, ISO string)
  • Covers compute_2d_subset_mask (full vs partial coverage)
  • Tests utils.assign_ugrid_topology (deprecation + delegation)

• Added tests/test_accessor.py:

  • Tests ds.xsg behavior when no grid is recognized:

    • Emits warning
    • subset_polygon and subset_bbox return None
    • subset_vars passthrough
    • has_vertical_levels behavior

• Updated tests/test_grids/test_sgrid.py:

  • Defines default zarr__version__ = 0 when optional dependencies are missing
  • Prevents test collection failures (NameError)

Fixes / Improvements

• Deprecation handling

  • Correct use of warnings.warn(...) with proper signature and stacklevel=2
  • Fixed deprecation message to reference assign_ugrid_topology correctly

• Input robustness

  • normalize_polygon_x_coords now coerces inputs using np.asarray(poly)
  • Ensures consistent behavior for list/tuple polygon inputs

Why this matters

• Improves coverage of previously untested core utilities
• Ensures consistent and predictable accessor behavior in edge cases
• Fixes incorrect deprecation warnings so users receive proper guidance
• Prevents CI/test failures when optional dependencies are unavailable
• Strengthens overall reliability and maintainability of the package

Notes

• This PR focuses on tests and small reliability fixes only
• Independent of README/documentation PR (docs/readme-quickstart-fixes)