Literals should raise a more friendly error message

[rambleraptor]

We should find a way to make sure that incorrect Literals raise a friendlier error message than the standard Pydantic error message. Pydantic might be able to do this for us.

Context: #2154 (comment)

[jayceslesar]

Nice, yeah looks like we can do something like

from typing import Any, Literal

from pydantic import BaseModel, Field, field_validator


class MyModel(BaseModel):
    my_literal: Literal[1] = Field(alias="my-literal")

    @field_validator("my_literal", mode="before")
    def validate_my_literal(cls, v: Any) -> int:
        """Validate that my_literal is 1 for MyModel."""
        if v != 1:
            raise ValueError(f"MyModel requires my-literal to be 1, got {v}")
        return v

MyModel(**{"my-literal": 2})

Which raises

pydantic_core._pydantic_core.ValidationError: 1 validation error for MyModel
my-literal
  Value error, MyModel requires my-literal to be 1, got 2 [type=value_error, input_value=2, input_type=int]
    For further information visit https://errors.pydantic.dev/2.11/v/value_error

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