Validation errors for Literal types incorrectly say 'enum value' #1009

Description

When validation fails for a typing.Literal type, msgspec raises a ValidationError with the message Invalid enum value <val>. This is misleading because the user is not using enum.Enum, they are using typing.Literal.

Reproduction

import msgspec
from typing import Literal

msgspec.json.decode(b'4', type=Literal[1, 2, 3])
# msgspec.ValidationError: Invalid enum value 4

msgspec.json.decode(b'"bad"', type=Literal["one", "two"])
# msgspec.ValidationError: Invalid enum value 'bad'

msgspec.json.decode(b'false', type=Literal[True])
# msgspec.ValidationError: Invalid enum value False

Expected

A wording that does not reference enum, since the type is not an enum. For example:

Invalid value 4
Invalid value 'bad'
Invalid value False

Or, even more useful, a message that lists the allowed literals:

Invalid value 4, expected one of (1, 2, 3)

Notes

• This affects all decode paths (JSON, MsgPack, convert).
• The same error path (_Lookup_OnMissing) is shared between Enum and Literal, so the fix needs to differentiate between the two cases (when lookup->cls != NULL it is an Enum, otherwise it is a Literal).
• Bool literals (added in Support Literal[True] and Literal[False] types #1004) use a separate code path but produce the same misleading wording.