fix issue with field_serializers on nested typed dicts

[davidhewitt]

Change Summary

This cleans up a bunch of FIXME comments I left regarding state.model handling in #1860.

It turns out for the most part that a lot of the stuff going on in fields.rs was redundant because model and dataclass serializers were already setting state.model. However for TypedDict, the code in fields.rs was supposed to be setting the model... but it was doing so extremely inconsistenty, and with problems for nested typed dicts.

The added test fails on main, passes the wrong typed dict to the nested field serializer.

I simplified the control flow by just adding a thin TypedDictSerializer wrapper which sets the state up in the same way as is done for models and dataclasses.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1879 will not alter performance

_{Comparing dh/state-model (d165e68) with main (20d576b)}

Summary

✅ 163 untouched