[modbus] Command lifecycle: PDU-carrying callbacks, on_sent(), notified queue clearing (#17886)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/components/modbus/common.h

@@ -0,0 +1,22 @@
+#pragma once
+#include <cstdint>
+#include "esphome/components/uart/uart_component.h"
+
+namespace esphome::modbus::testing {
+
+// A UART that discards all writes, for tests that never inspect the wire.
+class NullUART : public uart::UARTComponent {
+ public:
+  NullUART() { this->set_baud_rate(115200); }
+  void write_array(const uint8_t *data, size_t len) override {}
+  bool peek_byte(uint8_t *data) override { return false; }
+  bool read_array(uint8_t *data, size_t len) override { return false; }
+  size_t available() override { return 0; }
+  uart::UARTFlushResult flush() override { return uart::UARTFlushResult::UART_FLUSH_RESULT_ASSUMED_SUCCESS; }
+#if defined(USE_ESP8266) || defined(USE_ESP32)
+  void load_settings(bool dump_config) override {}
+#endif
+  void check_logger_conflict() override {}
+};
+
+}  // namespace esphome::modbus::testing