fix: retrieve pydantic example from json_schema_extra

[XanthanGum]

Add logic to retrieve example from json_schema_extra if not set.

Description

Pydantic models were not passing through examples via model_config = ConfigDict(json_schema_extra={'example':{'name':'XanthanGum'}}

Mentioned in #3181, point 6.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.18%. Comparing base (cbc0c51) to head (a731fe5).

Files with missing lines	Patch %	Lines
litestar/plugins/pydantic/utils.py	25.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5024      +/-   ##
==========================================
- Coverage   67.19%   67.18%   -0.02%     
==========================================
  Files         293      293              
  Lines       15363    15367       +4     
  Branches     1745     1747       +2     
==========================================
+ Hits        10323    10324       +1     
- Misses       4890     4891       +1     
- Partials      150      152       +2     

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