fix:Report the full buffer length to the UAC stack by setting bytes_read in usb_uac_device_input_cb(), so the host correctly detects received audio data instead of seeing zero bytes.

examples/peripherals/i2s/i2s_advance/i2s_usb/main/i2s_usb_example_main.c

@@ -236,6 +236,11 @@ static esp_err_t usb_uac_device_input_cb(uint8_t *buf, size_t len, size_t *bytes
         return ESP_FAIL;
     }
 
+    /* Report full-length data for UAC */
+    if (bytes_read) {
+        *bytes_read = len;
+    }
+
     return ESP_OK;
 }