feat(video): 事务化切换与前端统一编排，增强视频输入格式支持

- 后端：切换事务+transition_id，/stream/mode 返回 switching/transition_id 与实际 codec

- 事件：新增 mode_switching/mode_ready，config/webrtc_ready/mode_changed 关联事务

- 编码/格式：扩展 NV21/NV16/NV24/RGB/BGR 输入与转换链路，RKMPP direct input 优化

- 前端：useVideoSession 统一切换，失败回退真实切回 MJPEG，菜单格式同步修复

- 清理：useVideoStream 降级为 MJPEG-only

libs/hwcodec/cpp/ffmpeg_ram/ffmpeg_ram_encode.cpp

@@ -388,7 +388,9 @@ private:
     }
   _exit:
     av_packet_unref(pkt_);
-    return encoded ? 0 : -1;
+    // If no packet is produced for this input frame, treat it as EAGAIN.
+    // This is not a fatal error: encoders may buffer internally (e.g., startup delay).
+    return encoded ? 0 : AVERROR(EAGAIN);
   }
 
   int fill_frame(AVFrame *frame, uint8_t *data, int data_length,
@@ -511,4 +513,4 @@ extern "C" void ffmpeg_ram_request_keyframe(FFmpegRamEncoder *encoder) {
   } catch (const std::exception &e) {
     LOG_ERROR(std::string("ffmpeg_ram_request_keyframe failed, ") + std::string(e.what()));
   }
-}
+}