chore: enable ANN ruff ruleset for cohere integration

[julian-risch]

Related Issues

None

Proposed Changes:

• Add ANN (flake8-annotations) to ruff select in pyproject.toml
• Add ANN401 to ruff ignore (allow Any for SDK boundaries)
• Exclude tests/**/* from ANN checks via per-file-ignores
• Add -> None return type annotations to __init__ in text_embedder.py, document_embedder.py, generator.py, chat_generator.py, and ranker.py
• Add -> str return type annotation to EmbeddingTypes.__str__

How did you test it?

Ran tests locally

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.