test: use JSON unmarshalling for truncation test assertions

Replace fragile strings.Contains checks with proper JSON unmarshalling
to validate truncated fields by their exact values and verify URL fields
are correctly omitted.

rpc_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"errors"
 	"strings"
 
@@ -446,17 +447,34 @@ var _ = Describe("discordRPC", func() {
 			longAlbum := strings.Repeat("B", 200)
 			longURL := "https://example.com/" + strings.Repeat("x", 237)
 
+			truncatedName := strings.Repeat("N", 127) + "…"
+			truncatedTitle := strings.Repeat("T", 127) + "…"
+			truncatedArtist := strings.Repeat("A", 127) + "…"
+			truncatedAlbum := strings.Repeat("B", 127) + "…"
+
 			host.WebSocketMock.On("SendText", "testuser", mock.MatchedBy(func(msg string) bool {
-				// Text fields should be truncated to 128 runes (127 + "…")
-				truncatedName := strings.Repeat("N", 127) + "…"
-				truncatedTitle := strings.Repeat("T", 127) + "…"
-				truncatedArtist := strings.Repeat("A", 127) + "…"
-				truncatedAlbum := strings.Repeat("B", 127) + "…"
-				return strings.Contains(msg, truncatedName) &&
-					strings.Contains(msg, truncatedTitle) &&
-					strings.Contains(msg, truncatedArtist) &&
-					strings.Contains(msg, truncatedAlbum) &&
-					!strings.Contains(msg, longURL) // URL should be omitted
+				var message struct {
+					D json.RawMessage `json:"d"`
+				}
+				if err := json.Unmarshal([]byte(msg), &message); err != nil {
+					return false
+				}
+				var presence presencePayload
+				if err := json.Unmarshal(message.D, &presence); err != nil {
+					return false
+				}
+				if len(presence.Activities) != 1 {
+					return false
+				}
+				act := presence.Activities[0]
+				return act.Name == truncatedName &&
+					act.Details == truncatedTitle &&
+					act.State == truncatedArtist &&
+					act.Assets.LargeText == truncatedAlbum &&
+					act.DetailsURL == "" &&
+					act.StateURL == "" &&
+					act.Assets.LargeURL == "" &&
+					act.Assets.SmallURL == ""
 			})).Return(nil)
 
 			err := r.sendActivity("client123", "testuser", "token123", activity{