Replace all invariant list[ModelMessage] type hints with Sequence[ModelMessage]

[moritzwilksch]

Fixes #2940

Running this on the example from the issue works:

(pydantic-ai) ➜  pydantic-ai git:(mw/invarinat-typing-v2) ✗ uv run pyright mre.py

0 errors, 0 warnings, 0 informations

[DouweM]

@moritzwilksch Thanks for filing this. I think some of these changes are good, but many of them are unfortunately breaking changes (which we can't make until v2), as users may be treating the results of these methods and fields as a list, which it is now no longer guaranteed to be. It would also cause issues for people implementing their own Model subclass with an override for the request method with takes a messages argument that went from list to Sequence, as they'd now get type errors at the very least (invalid method override) and errors at worst (if they were treating it as a list). Same for people subclassing AbstractAgent and overriding the iter/run methods that take message_history.

Note that someone ran into issues because of #2797 as well: https://pydanticlogfire.slack.com/archives/C083V7PMHHA/p1757772153719419, but that was fine as it was pre-v1.

So I think we should reduce this to the smallest possible diff that addresses the user-facing issue you described in #2940, and then determine whether that would be a breaking change or not.

[github-actions]

This PR is stale, and will be closed in 3 days if no reply is received.

[github-actions]

Closing this PR as it has been inactive for 10 days.