Add chunking edge case tests

[lxingy3]

Description

Adds focused test coverage for chunking edge cases documented in #430:

• SentenceIterator start-position guards
• token interval helper error paths
• ChunkIterator constructor fallbacks
• TextChunk missing-document errors, sanitized text, and lazy caching
• batching with a short batch length
• broken_sentence reset behavior after a sentence is split mid-way

Fixes #430

Testing

How Has This Been Tested?

.venv\Scripts\python.exe -m pytest tests\chunking_test.py

Checklist:

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  or am covered by my company's
  Corporate CLA.
• I have discussed my proposed solution with code owners in the linked
  issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked
  issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed
  Google's Python Style Guide
  and ran pylint over the affected code.

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[lxingy3]

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