fix: fix movement of extras to json_schema_extra

[tlambert03]

there's a bug in the handling of extra kwargs to Fields.

This PR makes it so that **extras are added to json_schema_extra (i.e. with update), instead of the key metadata

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (dcb72bb) 94.69% compared to head (0c23625) 94.64%.

Files	Patch %	Lines
src/pydantic_compat/_v1/mixin.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   94.69%   94.64%   -0.06%     
==========================================
  Files           8        8              
  Lines         264      280      +16     
==========================================
+ Hits          250      265      +15     
- Misses         14       15       +1     

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