When we do our field type overrides to allow invocations to be instantiated without all required fields, we were not modifying the annotation of the field but did set the default value of the field to `None`.

This results in an error when doing a ser/de round trip. Here's what we end up doing:

```py
from pydantic import BaseModel, Field

class MyModel(BaseModel):
    foo: str = Field(default=None)
```

And here is a simple round-trip, which should not error but which does:

```py
MyModel(**MyModel().model_dump())
# ValidationError: 1 validation error for MyModel
# foo
#   Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
#     For further information visit https://errors.pydantic.dev/2.11/v/string_type
```

To fix this, we now check every incoming field and update its annotation to match its default value. In other words, when we override the default field value to `None`, we make its type annotation `<original type> | None`.

This prevents the error during deserialization.

This slightly alters the schema for all invocations and outputs - the values of all fields without default values are now typed as `<original type> | None`, reflecting the overrides.

This means the autogenerated types for fields have also changed for fields without defaults:

```ts
// Old
image?: components["schemas"]["ImageField"];

// New
image?: components["schemas"]["ImageField"] | null;
```

This does not break anything on the frontend.

This prevents issues where the node is defined with an invalid default value, which would guarantee an error during a ser/de roundtrip.

- Upstream issue requesting this functionality be built-in to pydantic: pydantic/pydantic#8722
- Upstream PR that implements the functionality: pydantic/pydantic-core#1593