feat: add support to return default if input is None

[Meetesh-Saini]

Change Summary

Added support to return default value when passing None as input.
Previously, passing None explicitly to a required field with a default value would raise a ValidationError

Related issue number

fix pydantic/pydantic#8972

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

please review

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.08%. Comparing base (ab503cb) to head (e5cf2ac).
Report is 207 commits behind head on main.

Files with missing lines	Patch %	Lines
src/validators/model_fields.rs	88.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1501      +/-   ##
==========================================
- Coverage   90.21%   89.08%   -1.13%     
==========================================
  Files         106      112       +6     
  Lines       16339    17890    +1551     
  Branches       36       40       +4     
==========================================
+ Hits        14740    15938    +1198     
- Misses       1592     1932     +340     
- Partials        7       20      +13     

Files with missing lines	Coverage Δ
python/pydantic_core/core_schema.py	94.85% <100.00%> (+0.08%)	⬆️
src/validators/model_fields.rs	94.01% <88.23%> (-2.46%)	⬇️

... and 49 files with indirect coverage changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #1501 will not alter performance

_{Comparing Meetesh-Saini:dev-none-default (e5cf2ac) with main (9b21b0f)}

Summary

✅ 155 untouched benchmarks

[davidhewitt]

I wonder if there is a simpler API here. Rather than going for config, given a BeforeValidator works here, can we do something simpler like:

class Foo(BaseModel):
    bar: Annotated[list[int], NoneAsDefault] = [0,1,2]
    baz: Annotated[list[str], NoneAsDefault] = ["a", "b", "c"]

... we might be able to do this purely in the pydantic side.

cc @sydney-runkle for opinions