feat(Modbus): add WriteMultipleRegistersAsync function

[ArgoZhang]

Link issues

fixes #25

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 implements the WriteMultipleRegistersAsync function for Modbus TCP client by adding support for multiple register writes. The implementation distinguishes between single and multiple register writes, using different packet formats according to the Modbus protocol.

• Added logic to handle multiple register writes with proper packet formatting
• Updated response validation to accommodate both single and multiple register operations
• Enhanced test cases with more comprehensive data arrays

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Longbow.Modbus/DefaultModbusTcpClient.cs	Implements multiple register write logic with conditional packet formatting and response validation
test/UnitTestModbus/TcpClientTest.cs	Updates test data arrays to include more values for testing multiple register scenarios

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[Copilot]

The response validation logic is incorrect. For multiple register writes, response.Span[10..11] should be response.Span[10..12] to properly compare the 2-byte quantity field, and data.Span[2..3] should be data.Span[2..4] to match the corresponding 2-byte quantity in the request data.

[Copilot]

Array index calculation is incorrect. The second line should use data[i * 2 + 6] which will cause an IndexOutOfRangeException. It should be data[i * 2 + 5] for the high byte and data[i * 2 + 6] for the low byte, but the array size calculation needs to account for this properly.

[Copilot]

The octal literal 01 should be written as 1 for consistency with other decimal values in the array, unless octal representation is specifically intended.

Suggested change

	var response = await client.WriteMultipleRegistersAsync(0x01, 0, [12, 0, 23, 0, 46, 0, 01, 02, 04, 05]);
	var response = await client.WriteMultipleRegistersAsync(0x01, 0, [12, 0, 23, 0, 46, 0, 1, 02, 04, 05]);