More union serialization tidying

[sydney-runkle]

1. Try value[discriminator_key] or value.key depending on the type of value
2. Only raise discriminator not found warnings if we're at the top level of a union, following up on Unstable behavior for warning Failed to get discriminator value for tagged union serialization ? pydantic#10657

1 fixes this issue that @Kludex reported, the following now runs with no warnings

from typing import Literal, Annotated
from pydantic import TypeAdapter, Discriminator
from typing_extensions import TypedDict

class ModelA(TypedDict):
    a: str
    state: Literal['ModelA']

class ModelB(TypedDict):
    b: str
    state: Literal['ModelB']

ta = TypeAdapter(Annotated[ModelA | ModelB, Discriminator('state')])
model_a = ta.validate_python({'a': 'a', 'state': 'ModelA'})
model_b = ta.validate_python({'b': 'b', 'state': 'ModelB'})
ta.dump_python(model_a)
ta.dump_python(model_b)

2 fixes pydantic/pydantic#10657

I've given some thought to how we might want to do a refactor here to minimize shared logic. I'm a bit held up on conflicting function signatures, so perhaps we could chat about that on Monday @davidhewitt :).

[codspeed-hq]

CodSpeed Performance Report

Merging #1536 will not alter performance

_{Comparing typed-dict-discriminator-warning (132dc48) with main (2419981)}

Summary

✅ 155 untouched benchmarks

[davidhewitt]

LGTM 👍

Regarding the shared logic, happy to chat about that separately & save that for a follow up refactor.