Fix serialization schema generation when using PlainValidator

[Viicos]

Do not hardcode a new wrap serializer schema if a serialization schema was already generated before.

Note that the following can be used as a failing MRE, related to the TODO comment:

from typing import Annotated

from pydantic import BaseModel, PlainValidator
from pydantic_core import core_schema

class MyDict:
    @classmethod
    def __get_pydantic_core_schema__(cls, source, handler):
        return core_schema.dict_schema(
            keys_schema=handler.generate_schema(str),
            values_schema=handler.generate_schema(int),
            serialization=core_schema.filter_dict_schema(
                include={'a'},
            ),
        )


class Model(BaseModel):
    f: Annotated[MyDict, PlainValidator(val)]

# If core schema validation is disabled:
Model(f={'a': 1, 'b': 1}).model_dump()
#> {'f': {'a': 1, 'b': 1}}
# Should be: {'f': {'a': 1}}

Maybe we should create a new issue, and add a xfail test linking to this issue?

Related issue number

Fixes #10385
Fixes #8586

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

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CodSpeed Performance Report

Merging #10427 will not alter performance

_{Comparing 10385 (1fb016f) with main (cf671c8)}

Summary

✅ 49 untouched benchmarks

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[sydney-runkle]

Do we not need to do handler.generate_schema(source_type) here?

[Viicos]

Hum I don't think it will change anything in this case as schema is guaranteed to not have any serialization key here, but I'll change to align with how it is done in Plain/WrapSerializer.

[sydney-runkle]

Wonderful, thanks @Viicos!!