Improve error handling for in-evaluable refs for discriminator application

[sydney-runkle]

This was supposed to close #8271, but right now it just improves the associated error.

My current hunch is that we need to do an apply discriminators step at the end of json schema building, like we do for core schemas, if we want this to work 😭. Not sure how worth the investment that is, at the moment.

I'd also be open to more creative solutions for evaluating refs for which we have not yet generated the schemas. Probably a good conversation to have with @Viicos and @adriangb.

MRE, at the moment:

from enum import Enum
from typing import Annotated, Union, Literal, List

from pydantic import BaseModel as BaseModel, Field, TypeAdapter
from pydantic.fields import FieldInfo
import json
from pydantic._internal._core_utils import pretty_print_core_schema

class ItemType(str, Enum):
    ITEM1 = 'item1'
    ITEM2 = 'item2'

class CreateItem1(BaseModel):
    item_type: Annotated[Literal[ItemType.ITEM1], Field(alias='type')]
    id: int

class CreateItem2(BaseModel):
    item_type: Annotated[Literal[ItemType.ITEM2], Field(alias='type')]
    id: int

class CreateObjectDto(BaseModel):
    id: int
    items: List[
        Annotated[
            Union[
                CreateItem1,
                CreateItem2,
            ],
            Field(discriminator='item_type'),
        ]
    ]

ta = TypeAdapter(
    Annotated[CreateObjectDto, FieldInfo(examples=[{'id': 1, 'items': [{'id': 3, 'type': 'ITEM1'}]}])]
)

print(json.dumps(ta.json_schema(), indent=2))
print(pretty_print_core_schema(ta.core_schema))

[codspeed-hq]

CodSpeed Performance Report

Merging #10440 will not alter performance

_{Comparing disc-fix-alias (113414a) with main (eda3eb4)}

Summary

✅ 38 untouched benchmarks

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

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[sydney-runkle]

This is done in the self.get_cache_defs_ref_schema(core_ref) call, so no reason to do that here.

[sydney-runkle]

This function handles the above removed logic more gracefully, with an intuitive error.

[Viicos]

I'll try to do a deep dive in the JSON Schema generation process to see what this is about, I'm guessing it is indeed pretty similar to the core schema gen process

[sydney-runkle]

Note from my conversation with @adriangb - we could alternatively do a first pass to collect all defs