Add support for enum as root codec (#6)

Author: kajebiii

core/src/main/scala/proteus/ProtobufCodec.scala

@@ -506,6 +506,7 @@ object ProtobufCodec {
         case c: Message[_]          => handleMessage(c, registers, offset)
         case c: RecursiveMessage[_] => handleMessage(c.codec, registers, offset)
         case c: Transform[_, _]     => c.from(loop(c.codec, offset))
+        case c: Enum[_]             => c.valuesByIndex(input.readEnum())
         case _                      => throw new Exception(s"Invalid root codec: $codec")
       }
 

core/src/test/scala/proteus/ProtobufCodecSpec.scala

@@ -92,6 +92,23 @@ object ProtobufCodecSpec extends ZIOSpecDefault {
 
         assert(results.map(_.status))(equalTo(variants))
       },
+      test("enum as root codec encoding/decoding") {
+        enum Status derives Schema { case Active, Inactive, Pending }
+
+        // Create a direct codec for the enum (this tests the new Enum case in loop function)
+        val enumCodec = Schema[Status].derive(deriver)
+
+        val testCases = List(Status.Active, Status.Inactive, Status.Pending)
+
+        val results = testCases.map { enumValue =>
+          val encoded = enumCodec.encode(enumValue)
+          val decoded = enumCodec.decode(encoded)
+          decoded == enumValue
+        }
+
+        // Verify all enum values can be used as root codec
+        assert(results.forall(identity))(isTrue)
+      },
       test("oneOf field variants") {
         enum ContactInfo derives Schema {
           case Email(address: String)