.NET: Change type of props representing continuation token

[SergeyMenshykh]

This PR changes type of properties representing continuation token from object? to ResponseContinuationToken?.

Contributes to: #95

[Copilot]

Pull request overview

This PR improves type safety by changing continuation token properties from object? to ResponseContinuationToken? throughout the agent framework.

Key Changes

• Changed property types in AgentRunOptions, AgentRunResponse, and AgentRunResponseUpdate from object? to ResponseContinuationToken?
• Removed unnecessary casts in ChatClientAgent and AgentRunResponseExtensions that are no longer needed due to the type change
• Updated sample code, tests, and workflow examples to use the new strongly-typed properties

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
AgentRunOptions.cs	Changed ContinuationToken property type from object? to ResponseContinuationToken?
AgentRunResponse.cs	Changed ContinuationToken property type from object? to ResponseContinuationToken?
AgentRunResponseUpdate.cs	Changed ContinuationToken property type from object? to ResponseContinuationToken?
AgentRunResponseExtensions.cs	Removed unnecessary casts when assigning continuation tokens
ChatClientAgent.cs	Removed unnecessary cast when assigning continuation token to chat options
AgentRunOptionsTests.cs	Updated test to create ResponseContinuationToken instance instead of generic object
10_Sequential_HostAsAgent.cs	Updated variable type from object? to ResponseContinuationToken?
11_Concurrent_HostAsAgent.cs	Updated variable type from object? to ResponseContinuationToken?
12_HandOff_HostAsAgent.cs	Updated variable type from object? to ResponseContinuationToken?
Agent_Step13_BackgroundResponsesWithToolsAndPersistence/Program.cs	Updated parameter and variable types from object? to ResponseContinuationToken?