feat(Modbus): add WriteRegisterAsync function

[ArgoZhang]

Link issues

fixes #21

Summary By Copilot

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• Merge the latest code from the main branch

[Copilot]

Pull Request Overview

This PR adds a new WriteRegisterAsync function to the Modbus TCP client implementation, along with updating existing register writing methods to return boolean success indicators instead of void tasks.

• Replaces WriteSingleRegisterAsync with WriteRegisterAsync returning ValueTask<bool>
• Updates WriteMultipleRegistersAsync to return ValueTask<bool> instead of Task
• Adds implementation for writing single and multiple registers with proper byte serialization

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/Longbow.Modbus/IModbusTcpClient.cs	Updates interface method signatures to return boolean success indicators
src/Longbow.Modbus/DefaultModbusTcpClient.cs	Implements new register writing methods and fixes byte array indexing bug
test/UnitTestModbus/TcpClientTest.cs	Adds unit tests for the new WriteRegisterAsync functionality

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[Copilot]

The byte array size calculation is incorrect. For multiple register writes, the data should be 4 + values.Length * 2 bytes (2 bytes for address + 2 bytes for count + 2 bytes per value), not 5 + values.Length * 2.

Suggested change

	var data = new byte[5 + values.Length * 2];
	data[0] = (byte)(address >> 8);
	data[1] = (byte)address;
	for (var i = 0; i < values.Length; i++)
	{
	data[i * 2 + 2] = (byte)(values[i] >> 8);
	data[i * 2 + 3] = (byte)(values[i] & 0xFF);
	var data = new byte[4 + values.Length * 2];
	data[0] = (byte)(address >> 8);
	data[1] = (byte)address;
	data[2] = (byte)(values.Length >> 8); // count high byte
	data[3] = (byte)(values.Length & 0xFF); // count low byte
	for (var i = 0; i < values.Length; i++)
	{
	data[i * 2 + 4] = (byte)(values[i] >> 8);
	data[i * 2 + 5] = (byte)(values[i] & 0xFF);

[Copilot]

The WriteUShortValues method is missing the quantity field that's required for multiple register writes. After the address bytes, it should include 2 bytes for the number of registers being written before the data bytes.

Suggested change

	var data = new byte[5 + values.Length * 2];
	data[0] = (byte)(address >> 8);
	data[1] = (byte)address;
	for (var i = 0; i < values.Length; i++)
	{
	data[i * 2 + 2] = (byte)(values[i] >> 8);
	data[i * 2 + 3] = (byte)(values[i] & 0xFF);
	// Modbus "Write Multiple Registers" request:
	// [0-1] Address
	// [2-3] Quantity of registers
	// [4] Byte count
	// [5...] Data (2 bytes per register)
	var quantity = (ushort)values.Length;
	var byteCount = (byte)(quantity * 2);
	var data = new byte[2 + 2 + 1 + quantity * 2];
	data[0] = (byte)(address >> 8);
	data[1] = (byte)address;
	data[2] = (byte)(quantity >> 8);
	data[3] = (byte)quantity;
	data[4] = byteCount;
	for (var i = 0; i < quantity; i++)
	{
	data[5 + i * 2] = (byte)(values[i] >> 8);
	data[5 + i * 2 + 1] = (byte)(values[i] & 0xFF);

[Copilot]

[nitpick] The comment is in Chinese and describes reading input registers, but the code is actually writing to a register. The comment should be corrected to describe the write operation and should be in English for consistency.

Suggested change

	// 读取 0x01 从站输入寄存器数据
	// Write value 12 to register 0 of slave 0x01

[Copilot]

[nitpick] The comment is in Chinese and describes reading input registers, but the code is actually writing multiple registers. The comment should be corrected to describe the write operation and should be in English for consistency.

Suggested change

	// 读取 0x01 从站输入寄存器数据
	// Write multiple registers to slave 0x01