Support Root models with Literal root types as discriminator field types

[YassinNouh21]

Summary

• Allow using RootModel[Literal[...]] as a discriminator field type in discriminated unions
• This is useful when using code generators like datamodel-code-generator that wrap Literal types in RootModel classes

The Problem

When using a RootModel[Literal['X']] as a field type for a discriminator, Pydantic previously failed with:

pydantic.errors.PydanticUserError: Model 'OperationActionData' needs field 'action' to be of type `Literal`

The Fix

Added handling in _infer_discriminator_values_for_inner_schema for 'model' schema type with 'root_model': True flag. This unwraps the RootModel to get the inner Literal schema.

Test plan

• Added test_discriminated_union_with_root_model_literal - basic RootModel[Literal] discriminator
• Added test_discriminated_union_with_root_model_literal_multiple_values - RootModel with multiple Literal values
• All existing discriminated union tests pass

Fixes #12605

[codspeed]

CodSpeed Performance Report

Merging this PR will improve performance by 6.4%

_{Comparing YassinNouh21:fix/root-model-literal-discriminator-12605 (547bbe3) with main (b6dae31)}

Summary

⚡ 1 improved benchmark
✅ 211 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_arguments	25.6 µs	24.1 µs	+6.4%

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_discriminated_union.py
Project Total

This report was generated by python-coverage-comment-action

[YassinNouh21]

@Viicos I will be free the next weeks, I would like to contribute to Paydantic so feel free to assign me to issues