Fix regex_engine being rejected by validate_core_schema

[davidhewitt]

Change Summary

Makes it actually possible to set regex_engine in pydantic/pydantic#7768

Related issue number

Part of pydantic/pydantic#7714

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

[davidhewitt]

In the interest of time today I have just pushed this missing item, I'd like to follow up with a proper test next week. 🙏

[sydney-runkle]

LGTM 👍

[codecov]

Codecov Report

Merging #1011 (82e57a4) into main (8bbfb33) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1011   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         106      106           
  Lines       15713    15714    +1     
  Branches       35       35           
=======================================
+ Hits        14621    14622    +1     
  Misses       1085     1085           
  Partials        7        7           

Files	Coverage Δ
python/pydantic_core/core_schema.py	94.75% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #1011 will improve performances by 14.19%

_{Comparing dh/core-schema-regex-engine (82e57a4) with main (8bbfb33)}

Summary

⚡ 3 improvements
✅ 137 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/core-schema-regex-engine	Change
⚡	test_variable_tuple	38.5 µs	34.3 µs	+12.46%
⚡	test_validate_literal[python-few_mixed]	33.1 µs	29 µs	+14.19%
⚡	test_validate_literal[json-few_mixed]	35.3 µs	31.4 µs	+12.53%