Add require_grounding parameter to filter ungrounded extractions

[ryankSD]

Fixes #209

Description

LangExtract extracts content from few-shot examples instead of just the input document. These hallucinated extractions have char_interval: None because they can't be located in the source text.

This PR adds a require_grounding parameter to extract() that filters out extractions where char_interval is None.

How Has This Been Tested?

• Added 8 unit tests in tests/test_require_grounding.py
• All new tests pass
• Existing test suite unaffected (380 tests still pass)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests that prove my fix works
• New and existing unit tests pass locally

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[ryankSD]

Since PR #392 has been closed, I've updated this branch to the latest main. All checks are passing and it's ready for review if this is still the preferred path for fixing #209.

[github-actions]

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