[zigbee_proxy] Auto-detect USB UART channel from uart_id, drop usb_uart_id

The usb_uart_id key was redundant: uart_id already points at the channel.
A new usb_uart.is_usb_uart_channel() helper checks the config tree (use_id
resolution does not narrow the ID type), and zigbee_proxy uses it to enable
the RX callback fast path and USB timeout defaults automatically.

tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml

@@ -0,0 +1,21 @@
+wifi:
+  ssid: test
+  password: password
+  power_save_mode: none
+
+api:
+
+usb_host:
+
+usb_uart:
+  - type: CDC_ACM
+    vid: 0x303A
+    pid: 0x831A
+    channels:
+      - id: zigbee_usb_channel
+        baud_rate: 460800
+
+# uart_id points at a USB UART channel; the component must detect this
+# automatically and enable the RX callback fast path (USE_ZIGBEE_PROXY_USB_UART)
+zigbee_proxy:
+  uart_id: zigbee_usb_channel