fix: enable locks for threadsafe pdfium

[dolfim-ibm]

This PR attempts to make the usage of Docling threadsafe, e.g. when running asynchronous jobs. See https://pypdfium2.readthedocs.io/en/v4/python_api.html#thread-incompatibility.

This is inspired by the langchain implementation, see https://python.langchain.com/api_reference/_modules/langchain_community/document_loaders/parsers/pdf.html.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

[mergify]

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[dolfim-ibm]

CI cancelled. It seems to have a deadlock.