chore: update dependency pin (rag-core-lib) to document-extractor service

[a-klos]

This pull request adds support for updating the rag-core-lib dependency pin in the document-extractor service's pyproject.toml and introduces a corresponding test to ensure this functionality works as expected.

Dependency management improvements:

• Updated the SERVICE_PINS mapping in bump_pyproject_deps.py to include rag-core-lib for the document-extractor service, allowing its version pin to be updated alongside extractor-api-lib.

Testing enhancements:

• Added a new test, test_bump_updates_service_pins, to verify that both extractor-api-lib and rag-core-lib version pins are correctly updated in the service's pyproject.toml when bump_service_pins=True is used.