.NET: Implement Task support for A2A Hosting package

[DeagleGross]

PR adds the support to A2A Tasks per protocol. As before it uses A2A SDK to subscribe to the events of receiving task/message updates and creation:

        taskManager.OnMessageReceived += OnMessageReceivedAsync;
        taskManager.OnTaskUpdated += OnTaskUpdatedAsync;
        taskManager.OnTaskCancelled += OnTaskCancelledAsync;

Task sessions are stored in the AgentSessionStore, and continuation token for the task is stored in the session-metadata.

Closes #406

[Copilot]

Pull request overview

This PR adds comprehensive Task support to the A2A Hosting package, implementing the A2A protocol's Task lifecycle management capabilities for long-running agent operations.

Changes:

• Introduces A2AResponseMode enum to control whether responses are returned as AgentMessage (lightweight), AgentTask (stateful), or determined automatically
• Implements full task lifecycle with OnTaskUpdated and OnTaskCancelled handlers for background operation polling
• Adds continuation token persistence in task metadata for resumable long-running operations
• Introduces A2AHostingJsonUtilities for AOT-friendly serialization with proper resolver chaining

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/AIAgentExtensions.cs	Core implementation of task support with OnMessageReceived, OnTaskUpdated, and OnTaskCancelled handlers; continuation token storage/retrieval logic
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AResponseMode.cs	New enum defining three response modes: Auto (default), Message, and Task
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AHostingJsonUtilities.cs	JSON serialization utilities with proper TypeInfoResolver chaining for AOT support
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/Converters/AdditionalPropertiesDictionaryExtensions.cs	Updated to use A2AHostingJsonUtilities instead of A2AJsonUtilities for consistency
dotnet/src/Microsoft.Agents.AI.Hosting.A2A.AspNetCore/EndpointRouteBuilderExtensions.cs	Added overloads accepting responseMode parameter across all MapA2A extension methods
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/AIAgentExtensionsTests.cs	Comprehensive test coverage for all three response modes, task lifecycle, continuation token handling, error scenarios, and cancellation