[AI assisted] MM-63304: Wrap around dqPtr while unmarshalling (#30371)

If the deadQueue was full, we would be incorrectly sending
the wrong index for dqPtr. Fixed that and added a test case.

https://mattermost.atlassian.net/browse/MM-63304

```release-note
NONE
```

server/channels/app/platform/websocket_reliable_test.go

@@ -65,3 +65,54 @@ func TestMarshalDQ(t *testing.T) {
 	assert.Equal(t, 3, dqPtr)
 	assert.Equal(t, events[:3], gotEvents[:3])
 }
+
+func TestUnmarshalDQFullBuffer(t *testing.T) {
+	ps := PlatformService{}
+
+	t.Run("dq full", func(t *testing.T) {
+		// Create exactly deadQueueSize events
+		events := make([]*model.WebSocketEvent, deadQueueSize)
+		for i := 0; i < deadQueueSize; i++ {
+			events[i] = model.NewWebSocketEvent(model.WebsocketEventPosted, "t1", "c1", "u1", nil, "").SetSequence(int64(i))
+		}
+
+		// Set up a scenario where the buffer is already filled and has wrapped around
+		// Use index 0 and simulate that the dqPtr has wrapped around to 0 again
+		got, err := ps.marshalDQ(events, 0, 0)
+		require.NoError(t, err)
+		require.Len(t, got, deadQueueSize)
+
+		// Unmarshal the full buffer back
+		gotEvents, dqPtr, err := ps.UnmarshalDQ(got)
+		require.NoError(t, err)
+
+		// Check that dqPtr wraps around to 0, not deadQueueSize
+		assert.Equal(t, 0, dqPtr, "dqPtr should be 0 for a full buffer (deadQueueSize % deadQueueSize = 0)")
+
+		// Verify all events were unmarshaled correctly
+		assert.Equal(t, events, gotEvents)
+	})
+
+	t.Run("dq rollover", func(t *testing.T) {
+		// Alternative test: Create a simulation of the circular buffer behavior
+		// This test fills up to the max and ensures wraparound works correctly
+		events := make([]*model.WebSocketEvent, deadQueueSize)
+		for i := 0; i < deadQueueSize; i++ {
+			// Create events with sequence numbers that show wraparound
+			// Last event will have highest sequence to demonstrate the break condition
+			// Seq nos: 100 - 228
+			events[i] = model.NewWebSocketEvent(model.WebsocketEventPosted, "t1", "c1", "u1", nil, "").SetSequence(int64(i + 100))
+		}
+
+		// Marshal only the last entry wrapping to the first to test wraparound detection
+		got2, err := ps.marshalDQ(events, deadQueueSize-1, 0)
+		require.NoError(t, err)
+		require.Len(t, got2, 1) // Just the last element
+
+		// Unmarshal this single element
+		gotEvents2, dqPtr2, err := ps.UnmarshalDQ(got2)
+		require.NoError(t, err)
+		assert.Equal(t, 1, dqPtr2, "dqPtr should be 1 for a 1-element buffer (1 % deadQueueSize = 1)")
+		assert.Equal(t, events[deadQueueSize-1], gotEvents2[0])
+	})
+}