Restore azure-functions dependency for Azure Functions apps

[pamelafox]

Purpose

The azure-functions package was accidentally removed from requirements.in in PR #2835. This package is required because the Azure Function apps (document_extractor, figure_processor, text_processor) reuse the backend's requirements.txt via the copy_prepdocslib.py script, and they need the azure-functions package for the Azure Functions runtime (import azure.functions as func).

Does this introduce a breaking change?

[ ] Yes
[x] No

Does this require changes to learn.microsoft.com docs?

[ ] Yes
[x] No

Type of change

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

Code quality checklist

See CONTRIBUTING.md for more details.

• The current tests all pass (python -m pytest).
• I added tests that prove my fix is effective or that my feature works
• I ran python -m pytest --cov to verify 100% coverage of added lines
• I ran python -m mypy to check for type errors
• I either used the pre-commit hooks or ran ruff and black manually on my code.

[mattgotteiner]

my bad