Enhance JSON payload decoding to accept both typed and pointer payloads, and add corresponding tests

internal/normalizers/common/json.go

@@ -18,6 +18,18 @@ import (
 // Errors include a small amount of operation context ("extract payload", "decode raw payload").
 // Callers typically wrap these with a provider/kind label.
 func DecodeJSONPayload[T any](in event.Event) (T, error) {
+	var zero T
+
+	if typed, ok := in.Payload.(T); ok {
+		return typed, nil
+	}
+	if ptr, ok := in.Payload.(*T); ok {
+		if ptr == nil {
+			return zero, fmt.Errorf("extract payload: payload pointer is nil")
+		}
+		return *ptr, nil
+	}
+
 	return fknormalize.DecodeJSONPayload[T](in)
 }