[modbus] Simplify payload size validation in modbus_helpers (#15838)

tests/components/modbus/modbus_helpers_test.cpp

@@ -0,0 +1,22 @@
+#include <gtest/gtest.h>
+
+#include "esphome/components/modbus/modbus_helpers.h"
+
+namespace esphome::modbus::helpers {
+
+TEST(ModbusHelpersTest, PayloadToNumberRejectsOffsetAtEndOfBuffer) {
+  const std::vector<uint8_t> data{0x12, 0x34};
+  EXPECT_EQ(payload_to_number(data, SensorValueType::U_WORD, 2, 0xFFFFFFFF), 0);
+}
+
+TEST(ModbusHelpersTest, PayloadToNumberRejectsTruncatedMultiRegisterValue) {
+  const std::vector<uint8_t> data{0x12, 0x34, 0x56};
+  EXPECT_EQ(payload_to_number(data, SensorValueType::U_DWORD, 0, 0xFFFFFFFF), 0);
+}
+
+TEST(ModbusHelpersTest, PayloadToNumberDecodesValidWord) {
+  const std::vector<uint8_t> data{0x12, 0x34};
+  EXPECT_EQ(payload_to_number(data, SensorValueType::U_WORD, 0, 0xFFFFFFFF), 0x1234);
+}
+
+}  // namespace esphome::modbus::helpers