Warn when string input matches an existing file path

[UditDewan]

Description

extract() treats string input as literal text. When a user passes a file path instead of the file's contents (an easy mistake), the path itself is sent to the model, which can echo the local directory structure and OS username into extraction results, or hallucinate content for the "document" — exactly the exposure reported in the linked issue.

This PR makes extract() emit a UserWarning when text_or_documents is a string that matches a path to an existing file, telling the user that the path (not the file contents) will be sent to the model and showing how to read the file first. The check is guarded to non-URL strings under 4096 chars, so normal document text never pays a filesystem stat. Behavior is otherwise unchanged: strings are still never read from disk, keeping the API free of any implicit file access. The text_or_documents docstring now documents this.

Fixes #249

Bug fix

How Has This Been Tested?

Added FilePathInputWarningTest to tests/init_test.py: a string matching an existing file path emits the warning; ordinary text does not.

$ python -m pytest tests/init_test.py
27 passed, 18 subtests passed

$ pylint --rcfile=tests/.pylintrc tests/init_test.py
Your code has been rated at 10.00/10

$ isort langextract tests && pyink langextract tests --config pyproject.toml
2 files left unchanged.

The full suite (pytest tests --ignore=tests/test_live_api.py) shows the same results as unmodified main in my environment, plus the two new tests.

Checklist:

• I have read and acknowledged Google's Open Source
  Code of conduct.
• I have read the
  Contributing
  page, and I either signed the Google
  Individual CLA
  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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[github-actions]

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