enhancement: raise notimplementederror on single index rather than random error

[qued]

Note

Explicitly raises NotImplementedError when single-indexing TextRegions; adds a skipped test, updates changelog, and bumps version to 1.1.1.

• Elements:
  • unstructured_inference/inference/elements.py: Single-element indexing of TextRegions now raises NotImplementedError in slice() (used by __getitem__).
• Tests:
  • test_unstructured_inference/test_elements.py: Adds a skipped test covering single-element access expectations.
• Release:
  • __version__ bumped to 1.1.1.
  • CHANGELOG.md: Notes the new NotImplementedError behavior for single indexing.

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