Fix standalone enum registration in ObjectSerializer enumsMap

Replace runtime enum value entries (e.g. Result.Valid) with the type
name string (e.g. "Result") in the enumsMap set, so enumsMap.has(type)
correctly identifies standalone enums during serialization/deserialization.

Also update the Mustache template to skip importing standalone enum types
since they are no longer referenced in the generated objectSerializer body.

Fixes #1659