feat(slave_node): Refine Modbus UART and add CDC-ACM support

- Adjusted Device Tree Overlays for bluepill_f103rb and weact_stm32g431_core
  to correctly define Modbus UART via 'modbus0' subnode with 'zephyr,modbus-serial'
  compatibility, aligning with rtu_server sample.
- Prepared modbus_server.c to use the correct Device Tree node for Modbus UART.

software/apps/slave_node/boards/bluepill_f103rb.overlay

@@ -12,6 +12,7 @@
         label = "RTT";
         status = "okay";
     };
+
 };
 
 &flash0 {
@@ -34,10 +35,13 @@
     };
 };
 
-&usart1 {
-	modbus0 {
-		compatible = "zephyr,modbus-serial";
-		status = "okay";
-	};
-	status = "okay";
-};
+ &usart1: {
+    pinctrl-0 = <&usart1_tx_pa9 &usart1_rx_pa10>;
+    pinctrl-names = "default";
+
+    modbus0 {
+        compatible = "zephyr,modbus-serial";
+        status = "okay";
+    };
+    status = "okay";
+};

software/apps/slave_node/boards/weact_stm32g431_core.overlay

@@ -1,9 +1,10 @@
 &usart1 {
-	modbus0 {
-		compatible = "zephyr,modbus-serial";
-		status = "okay";
-	};
-	status = "okay";
     pinctrl-0 = <&usart1_tx_pa9 &usart1_rx_pa10>;
     pinctrl-names = "default";
-};
+
+    modbus0 {
+        compatible = "zephyr,modbus-serial";
+        status = "okay";
+    };
+    status = "okay";
+};