feat(Modbus): add WriteCoilAsync function

[ArgoZhang]

Link issues

fixes #15

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 asynchronous single coil write functionality to the Modbus TCP client by implementing the WriteCoilAsync method. The changes simplify the interface by removing unused synchronous methods and focus on providing a clean async-only API.

• Implements WriteCoilAsync method for writing single coil values asynchronously
• Adds BuildWriteRequest method to support write operations in the message builder
• Removes unused synchronous methods and simplifies the interface

Reviewed Changes

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

File	Description
test/UnitTestModbus/TcpClientTest.cs	Adds unit test for the new WriteCoilAsync functionality
src/Longbow.Modbus/ModbusMessageBuilder.cs	Renames existing method and adds BuildWriteRequest for write operations
src/Longbow.Modbus/IModbusTcpClient.cs	Updates interface to include WriteCoilAsync and removes unused methods
src/Longbow.Modbus/DefaultModbusTcpClient.cs	Implements WriteCoilAsync and supporting write functionality

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