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

internal/normalizers/common/json_test.go

@@ -0,0 +1,39 @@
+package common
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+func TestDecodeJSONPayloadAcceptsTypedPayload(t *testing.T) {
+	type rawPayload struct {
+		Value string `json:"value"`
+	}
+
+	got, err := DecodeJSONPayload[rawPayload](event.Event{
+		Payload: rawPayload{Value: "ok"},
+	})
+	if err != nil {
+		t.Fatalf("DecodeJSONPayload() error = %v", err)
+	}
+	if got.Value != "ok" {
+		t.Fatalf("Value = %q, want ok", got.Value)
+	}
+}
+
+func TestDecodeJSONPayloadAcceptsTypedPointerPayload(t *testing.T) {
+	type rawPayload struct {
+		Value string `json:"value"`
+	}
+
+	got, err := DecodeJSONPayload[rawPayload](event.Event{
+		Payload: &rawPayload{Value: "ok"},
+	})
+	if err != nil {
+		t.Fatalf("DecodeJSONPayload() error = %v", err)
+	}
+	if got.Value != "ok" {
+		t.Fatalf("Value = %q, want ok", got.Value)
+	}
+}