fix: explicitly re-export DocumentStream from base_models

[stablegenius49]

Summary

• explicitly re-export DocumentStream from docling.datamodel.base_models
• add __all__ to make the module's public exports explicit for static type checkers
• add a regression test that checks DocumentStream stays part of the public export surface

Testing

• python3 -m py_compile docling/datamodel/base_models.py tests/test_input_doc.py
• attempted: python3 -m pytest tests/test_input_doc.py -k 'explicitly_exports_document_stream or valid_buf or invalid_buf'
  • could not run in this workspace because the local environment is missing repo dependencies such as pydantic

Fixes #3165
Related to #614

[mergify]

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[cau-git]

This appears to be a duplicate of #3181, therefore I am closing this one.