[Bug]: support mix types for enum types in tcgc

[ArcturusZhang]

Describe the bug

I have this enum:

enum Fixed {
	One: 1,
	Two: "2"
}

which is totally fine in typespec, but TCGC gives me string as its valueType, which is just incorrect.

Suggestion

I think it is reasonable that we return a union as its value type which is more intuitive.

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the TypeSpec repo
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.

[tadelesh]

emm, i doubt if it should be fixed in tcgc enum type or add a lint rule to avoid it.

[tadelesh]

@timotheeguerin is it a valid case for typespec?

[timotheeguerin]

it is today a valid use case, we don't have a way to limit enums types. But either way I would agree that we probably would want a linting rule against that for azure.

[tadelesh]

since we have lint in core that disallow azure service to have enums: https://github.com/Azure/typespec-azure/blob/main/packages/typespec-azure-core/src/rules/no-enum.ts, so i believe it is an only ask for unbranded. i finished the support pr, we could decide whether we need to have this breaking change in the sync up.