Shall we disable ```seal-single-value-enum-by-default``` for the generation?

[kazrael2119]

In this case:
https://github.com/Azure/sdk-release-request/issues/6319#issuecomment-3061041111
Not all cases require intervalUnit but our code set it with a default value, So the request failed.

intervalUnit is a enum that only has single value, and in swagger, this enum's modelAsString is false https://github.com/Azure/azure-rest-api-specs/blob/main/specification/storage/resource-manager/Microsoft.Storage/stable/2024-01-01/storageTaskAssignments.json#L578-L586
So the generated code will be like:

interface xxx {
   intervalUnit?: "Days"
}

and our codegen always enables seal-single-value-enum-by-default as true
So the property intervalUnit will have a default value "Days"

[qiaozha]

@kazrael2119 please verify if this case works in TypeSpec generation.
Please verify two things:

1. if single value fixed enum and extensible enum could be correctly generated.
2. If optional constant type could be correctly serialized and deserialization when that it's absent.

[kazrael2119]

for 1: fixed enum and extensible enum can be correctly generated
for 2:
it should be correctly