Validate default values at schema definition

[changhc]

Change Summary

Extended option validate_default so that it's possible to validate default values when a model schema is being built.

This option now can be a boolean or 3 possible string values: 'never', 'definition', or 'init'.

• never: no validation for default values at all. This is the same as False.
• init: validation at model instantiation. This is the same as True.
• definition: validation at model definition.

The behaviour of True/False does not change with this PR.

Note that even though init and definition are not mutually exclusive and I implemented these as flags that can co-exist, I decided not to add another option, e.g. always, that cover both flags because if a validation is raised at model definition, it's pretty much impossible to trigger such error at model instantiation because the model does not built at all and therefore we don't really need always.

Related issue number

Part of pydantic/pydantic#8722

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

Selected Reviewer: @davidhewitt

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.60%. Comparing base (ab503cb) to head (1a82948).
Report is 264 commits behind head on main.

Files with missing lines	Patch %	Lines
src/validators/with_default.rs	89.18%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1593      +/-   ##
==========================================
- Coverage   90.21%   89.60%   -0.62%     
==========================================
  Files         106      112       +6     
  Lines       16339    17909    +1570     
  Branches       36       40       +4     
==========================================
+ Hits        14740    16047    +1307     
- Misses       1592     1842     +250     
- Partials        7       20      +13     

Files with missing lines	Coverage Δ
python/pydantic_core/core_schema.py	94.86% <100.00%> (+0.09%)	⬆️
src/validators/with_default.rs	93.47% <89.18%> (-5.60%)	⬇️

... and 62 files with indirect coverage changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #1593 will not alter performance

_{Comparing changhc:8722-validate-default (1a82948) with main (562fad3)}

Summary

✅ 157 untouched benchmarks

[changhc]

please review

[psychedelicious]

Thank you! The problem this addresses has been a persistent thorn in our sides. Would love to see this released.