Enable mypy and resolve typing issues in OTX library

mypy detects >1k typing violations In `library/otx`. These errors were previously undetected due to a misconfiguration of pre-commit, that would cause mypy not to follow some imports.

To show all the mypy errors, run:
```shell
cd library
uv run mypy .
```

The goal is to gradually resolve all the errors and re-enable mypy in CI.