Only fetch __pydantic_core_schema__ from the current class during schema generation

[Viicos]

In some cases, the core schema of a class is not the right one:

from pydantic.dataclasses import dataclass as pd_dc

from dataclasses import dataclass as stdlib_dc

@pd_dc
class A:
    a: int

@stdlib_dc
class B(A):
    b: str

B.__pydantic_core_schema__
#> Core schema of `A`

Until now, we had the existing_schema.get('cls', None) is obj check to avoid incorrectly using the core schema in this case. However, this doesn't work if the core schema is of type definitions, in which case existing_schema.get('cls', None) is obj is False even though it is the correct schema. This was found during the ns refactor.

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• 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
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[Viicos]

We could also check for __slots__ but there's no such type with a __pydantic_core_schema__ property as of today, so I'm keeping things simple here by only checking for __dict__

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CodSpeed Performance Report

Merging #10518 will not alter performance

_{Comparing gen-schema-from-property-dict (8dff26d) with main (01b5929)}

Summary

✅ 38 untouched benchmarks

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[sydney-runkle]

Nice find!