👷 Add ty to the CI lint check

[svlandeg]

For now, running ty in parallel to mypy to see what kind of errors/warnings come up.

[github-actions]

📝 Docs preview

Last commit 5a7bf84 at: https://3cf779e6.typertiangolo.pages.dev

[svlandeg]

This edit and the lines below is not strictly necessary: they are "possibly-missing-attribute" warnings from ty that won't fail the CI. Still, IMO it's nice to test for this explicitely up-front instead of having the try-catch. This is a bit of personal preference though, so will leave it to Tiangolo to make the final decision.

[svlandeg]

Instead of making these edits, we have two other options:

• add ty: ignore statements
• just leave the warnings as is (though this is somewhat annoying when running the tool locally and getting all the warnings output)

[svlandeg]

Ok this is ready for review @tiangolo.

TLDR decisions to take:

• Do we resolve issues with assert and cast statements (first few commits in this PR) or by using ignore statements (final commits & current state)? I think mostly in our repo's we've been resorting to ignore statements so far.
• If we want ignore statements, are we happy with ty: ignore? I'd prefer it because 1) ty doesn't support type: ignore with specific violations between brackets, and 2) if we use type: ignore, mypy will complain, and we'd have to put type: ignore[unused-ignore] which will work because of 1) but feels ugly/hacky.
• Do we want to resolve also ty warnings even if they don't fail the CI? (if yes, we can also enforce this via the CI with --error-on-warning