[12.x] Implement Stringable in Enum rule

[owenconti]

This allows the Enum rule class to be stringable, serializing to the "in:" rule.

[alexbowers]

This PR has caused a breaking change in how Enums are produced in OpenAPI generation using Scramble.

Using Laravel 12.47.0 you get the following (truncated) output:

"AssignUserRoleResponseData": {
                "properties": {
                    "role": {
                        "$ref": "#/components/schemas/UserRole"
                    }
                },
                "required": [
                    "role"
                ],
                "title": "AssignUserRoleResponseData",
                "type": "object"
            },

After this PR, it now looks like this:

"AssignUserRoleResponseData": {
                "properties": {
                    "role": {
                        "enum": [
                            "admin",
                            "member",
                            "jobillan"
                        ],
                        "type": "string"
                    }
                },
                "required": [
                    "role"
                ],
                "title": "AssignUserRoleResponseData",
                "type": "object"
            },

When this is combined with Typegen generation, what this means is that previously where Enums would be exported and generated for us (we use Kubb for this), these are now inline, and the exported enums no longer are referrable in Typescript.

[alexbowers]

For anyone else who comes here with the same issue; Scramble have released a fix for this on their end v0.13.11