Fix EnumSerializer to honor use_enum_values (issue #10570)

[Akshay9715]

Summary

This change updates the EnumSerializer in pydantic-core to properly respect the use_enum_values configuration flag when serializing Python Enum types. Specifically:

• Adds a use_enum_values: bool field to the EnumSerializer struct.
• Reads use_enum_values from the model’s model_config in build() (defaulting to false).
• Updates to_python() and json_key() to unwrap the enum’s .value whenever use_enum_values is true, ensuring that nested models also serialize their enum fields as raw values.
• Leaves existing behavior unchanged when use_enum_values is false or unset.

These edits resolve the inconsistency described in Issue #10570 so that enums in nested contexts now correctly output their underlying values.

Related issue number

fix #10570

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

[Viicos]

Thanks for the contribution @Akshay9715, but AI won't help in solving these kind of complex issues. As per my analysis, this isn't the way to go.