[modbus] Add API for reading/writing coils and discrete inputs in server mode (#17264)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: J. Nick Koston <nick@koston.org>

tests/components/modbus_controller/command_payload_test.cpp

@@ -0,0 +1,31 @@
+#include <gtest/gtest.h>
+
+#include <cstdint>
+#include <vector>
+
+#include "esphome/components/modbus_controller/modbus_controller.h"
+
+namespace esphome::modbus_controller::testing {
+
+// The coil write factory packs into an exact-size payload. Pinned at one past the protocol maximum
+// because a fixed pack buffer sized for the maximum would silently truncate there while the quantity
+// field still claimed every coil - and the truncated frame would fit the RTU limit and go on the wire
+// malformed. Built at its true byte count, the oversize frame is refused by the hub's size check with
+// a log instead.
+TEST(ModbusCommandPayload, CoilWritePayloadIsExactSizedNotTruncated) {
+  ModbusController controller;
+  std::vector<bool> coils(modbus::MAX_NUM_OF_COILS_TO_WRITE + 1, true);
+  auto cmd = ModbusCommandItem::create_write_multiple_coils(&controller, 0x10, coils);
+  EXPECT_EQ(cmd.payload.size(), modbus::packed_bit_bytes(coils.size()));
+}
+
+// LSB-first packing with zeroed pad bits, matching the wire layout the PDU builders produce.
+TEST(ModbusCommandPayload, CoilWritePacksLsbFirstWithZeroPad) {
+  ModbusController controller;
+  const std::vector<bool> coils{true, false, true, true};
+  auto cmd = ModbusCommandItem::create_write_multiple_coils(&controller, 0x10, coils);
+  ASSERT_EQ(cmd.payload.size(), 1u);
+  EXPECT_EQ(cmd.payload.data()[0], 0b00001101);
+}
+
+}  // namespace esphome::modbus_controller::testing