fix borrow error when serializing recursive models

[davidhewitt]

Change Summary

Fixes a borrow error when serializing a recursive model.

The cause was because we took a &mut borrow on the SchemaSerializer in to_json but then attempt to clone (!!) it when we fetch __pydantic_serializer__.

I've changed to (a) not take a &mut borrow and (b) not clone when fetching __pydantic_serializer__.

Related issue number

pydantic/pydantic#6571

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: @samuelcolvin

[codspeed-hq]

CodSpeed Performance Report

Merging #764 will not alter performance

_{Comparing dh/pyd-issue-6571 (f136db9) with main (3e7cc4f)}

Summary

✅ 126 untouched benchmarks

[codecov]

Codecov Report

Merging #764 (f136db9) into main (3e7cc4f) will decrease coverage by 0.03%.
The diff coverage is 88.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #764      +/-   ##
==========================================
- Coverage   93.65%   93.63%   -0.03%     
==========================================
  Files          99       99              
  Lines       14148    14149       +1     
  Branches       25       25              
==========================================
- Hits        13250    13248       -2     
- Misses        892      895       +3     
  Partials        6        6              

Impacted Files	Coverage Δ
src/serializers/mod.rs	98.55% <80.00%> (-0.48%)	⬇️
src/serializers/infer.rs	96.89% <100.00%> (ø)
src/serializers/shared.rs	90.06% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[davidhewitt]

please review