Missing media_type

[mwildehahn]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

Creating a new issue with a failing test for this one: #2394.

Failing test is here: 56fc78c. I think this has to do with the _media_type being an attribute ImageUrl gets from FileUrl, but when we serialize, we don't include that property because of this: https://docs.pydantic.dev/latest/concepts/serialization/#subclass-instances-for-fields-of-basemodel-dataclasses-typeddict.

So when we deserialize and try and access _media_type we get the error that the property doesn't exist. I thought this only happened if media_type was missing but I added two tests one without it and expanded the test @DouweM had with it and both fail.

Example Code

Python, Pydantic AI & LLM client version

de000324

[mwildehahn]

you can see from the test on main right now that media type is lost: https://github.com/pydantic/pydantic-ai/pull/2422/files#diff-359c72157a993d9c94d8e17fd4591e8403abcfbe4b87507bfba7958e4edf8f7aR2993-R2997 and then when we deserialize, there is nothing there because there is no default.

I don't have full context on why _media_type was set with init=False, but removing that fixes the issue