fix: ensure to always return one of the values in Literal field type

[PrettyWood]

Change Summary

Ensure we always return a value set in Literal even though other values are equal (for example a StrEnum where a string can be equal to a enum member but not identical)

Related issue number

closes #2166

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2181 (6bd48f5) into master (de0657e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2181   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         4121      4122    +1     
  Branches       829       829           
=========================================
+ Hits          4121      4122    +1     

Impacted Files	Coverage Δ
pydantic/validators.py	100.00% <100.00%> (ø)

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