Stop invalid encoder output explicitly while preserving transient backpressure retries

tools/chatgui/src/call_video_encoder.cpp

@@ -123,6 +123,7 @@ bool CallVideoEncoder::encode(const QImage& image, uint64_t pts, bool forceKey,
         }
         if (m_switching && frame.pts == m_switchPts) {
             if (!frame.key) { m_error = "Replacement encoder did not start with KEY";
+                completion = {m_format.generation, m_request, 0};
                 DEBUG_ERROR(DEBUG_CATEGORY_CALL, "%s", qPrintable(m_error)); return false; }
             completion = {m_format.generation, m_request, m_pendingBitrate};
             DEBUG_INFO(DEBUG_CATEGORY_CALL, "call video bitrate committed generation=%llu request=%llu bitrate=%u pts=%llu background_ms=%lld",