[uart] Fix RTL87xx compilation failure due to SUCCESS macro collision

The Realtek SDK defines `#define SUCCESS 0` in basic_types.h which
collides with the FlushResult::SUCCESS enum value, breaking compilation
on RTL87xx devices when api/wifi components pull in the SDK headers.

Add `#undef SUCCESS` before the enum definition, following the same
pattern used elsewhere in the codebase for vendor SDK macro collisions.

Also add RTL87xx UART compilation test.

esphome/components/uart/uart_component.h

@@ -30,12 +30,17 @@ enum UARTDirection {
 const LogString *parity_to_str(UARTParityOptions parity);
 
 /// Result of a flush() call.
+// Some vendor SDKs (e.g., Realtek) define SUCCESS as a macro.
+// Save and restore around the enum to avoid collisions with our scoped enum value.
+#pragma push_macro("SUCCESS")
+#undef SUCCESS
 enum class FlushResult {
   SUCCESS,          ///< Confirmed: all bytes left the TX FIFO.
   TIMEOUT,          ///< Confirmed: timed out before TX completed.
   FAILED,           ///< Confirmed: driver or hardware error.
   ASSUMED_SUCCESS,  ///< Platform cannot report result; success is assumed.
 };
+#pragma pop_macro("SUCCESS")
 
 class UARTComponent {
  public:

tests/components/uart/test.rtl87xx-ard.yaml

@@ -0,0 +1,14 @@
+uart:
+  - id: uart_id
+    tx_pin: PA23
+    rx_pin: PA18
+    baud_rate: 9600
+    data_bits: 8
+    parity: NONE
+    stop_bits: 1
+
+switch:
+  - platform: uart
+    name: "UART Switch"
+    uart_id: uart_id
+    data: [0x01, 0x02, 0x03]