.NET: Add support for background responses to A2A agent

[SergeyMenshykh]

This PR adds support for AgentTask to the A2A agent and also includes support for background responses to the non-streaming RunAsync method, enabling consumers to poll for task completion.

The following items are out of scope for this PR and will be addressed separately in follow-up PRs:

• Support for input requests from the A2A agent
• Internal polling for agent task completion in the RunAsync method
• Background responses for the streaming API: RunStreamingAsync

Contributes to: #95

[Copilot]

Pull request overview

This pull request adds support for AgentTask and background responses to the A2A agent, enabling consumers to poll for long-running task completion using continuation tokens.

Key changes:

• Introduces A2AContinuationToken for tracking background task execution state
• Updates A2AAgentThread to store both conversation context ID and task ID
• Enhances A2AAgent.RunAsync to handle task responses and support polling via continuation tokens
• Adds comprehensive unit tests for the new functionality
• Includes a sample demonstrating polling for task completion

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Core implementation of task support, continuation token handling, and streaming task event processing
dotnet/src/Microsoft.Agents.AI.A2A/A2AContinuationToken.cs	New class implementing ResponseContinuationToken for task state tracking
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgentThread.cs	Enhanced to store task ID alongside context ID with proper serialization
dotnet/src/Microsoft.Agents.AI.A2A/A2AJsonUtilities.cs	New utility class providing JSON serialization configuration for A2A types
dotnet/src/Microsoft.Agents.AI.A2A/Extensions/A2AArtifactExtensions.cs	Refactored to extract ToAIContents method for better code reuse
dotnet/src/Microsoft.Agents.AI.A2A/Extensions/A2AAgentTaskExtensions.cs	Enhanced with new ToAIContents method and nullable return types for better null handling
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs	Extensive new tests covering continuation tokens, task responses, and streaming scenarios
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/Extensions/A2AArtifactExtensionsTests.cs	New unit tests for artifact extension methods
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/Extensions/A2AAgentTaskExtensionsTests.cs	New unit tests for agent task extension methods
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AContinuationTokenTests.cs	New comprehensive tests for continuation token serialization and deserialization
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentThreadTests.cs	New tests for thread state serialization
dotnet/samples/GettingStarted/A2A/A2AAgent_PollingForTaskCompletion/Program.cs	New sample demonstrating polling for long-running task completion
dotnet/src/Microsoft.Agents.AI.A2A/Microsoft.Agents.AI.A2A.csproj	Removed version overrides for package references and added trim attributes configuration
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/Microsoft.Agents.AI.A2A.UnitTests.csproj	Removed version overrides and added NoWarn for MEAI001