feat(Modbus): add ReadHoldingRegistersAsync function

[ArgoZhang]

Link issues

fixes #11

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 async reading functionality for Modbus holding registers and input registers, along with interface updates to support nullable return types and improved documentation.

Key changes include:

• Added ReadHoldingRegistersAsync and ReadInputRegistersAsync method implementations
• Updated interface method signatures to return nullable arrays and use ValueTask instead of Task
• Refactored code to use generic parsing functions for different data types

Reviewed Changes

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

File	Description
IModbusTcpClient.cs	Updated interface with nullable return types, consistent ValueTask usage, and improved Chinese documentation
DefaultModbusTcpClient.cs	Implemented async register reading methods with generic parsing approach and code refactoring
TcpClientTest.cs	Added comprehensive test coverage for the new async register reading methods

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