.NET: Add CreateSessionAsync overload with taskId for A2AAgent session resumption

[Copilot]

Motivation and Context

A2AAgent sessions track both ContextId (conversation) and TaskId (current task), but CreateSessionAsync only accepted contextId, preventing session resumption from a specific task.

Description

Added CreateSessionAsync(string contextId, string taskId) overload to enable resuming sessions with both conversation and task context. Both overloads use expression-bodied members with inlined parameter validation for a concise implementation.

Changes:

• API Addition: New overload creates A2AAgentSession with both IDs set
• Parameter Validation: Both overloads validate parameters using inlined Throw.IfNullOrWhitespace calls within object initializers
• Expression-Bodied Members: Both methods use lambda-style expression bodies for concise implementation
• Backward Compatibility: Existing CreateSessionAsync(string contextId) overload unchanged in functionality
• Tests: Added comprehensive coverage for both overloads including parameter validation tests (17 total tests)

Example:

// Resume conversation only (existing)
var session1 = await agent.CreateSessionAsync("context-123");

// Resume from specific task (new)
var session2 = await agent.CreateSessionAsync("context-123", "task-456");

// Both methods validate parameters and throw ArgumentException for null/empty/whitespace

Implementation:

public ValueTask<AgentSession> CreateSessionAsync(string contextId)
    => new(new A2AAgentSession() { ContextId = Throw.IfNullOrWhitespace(contextId) });

public ValueTask<AgentSession> CreateSessionAsync(string contextId, string taskId)
    => new(new A2AAgentSession() { ContextId = Throw.IfNullOrWhitespace(contextId), TaskId = Throw.IfNullOrWhitespace(taskId) });

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.NET: A2AAgent should support the ability to resume a session based on contextid and taskid</issue_title>
<issue_description>Currently the .net A2AAgent has a CreateSession overload that allows a user to provide a context id, however it does not allow the user to also provide a task id. It's important for users to also optionally be able to provide a task id, so that a session can be resumed from a specific task.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes .NET: A2AAgent should support the ability to resume a session based on contextid and taskid #3923

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Pull request overview

This PR adds an additional CreateSessionAsync overload to A2AAgent to enable session resumption with both context and task identifiers. Previously, only the context ID could be specified when resuming a session, but A2A protocol sessions track both ContextId (conversation) and TaskId (current task). This change enables users to resume from a specific task within a conversation.

Changes:

• Added CreateSessionAsync(string contextId, string taskId) overload to A2AAgent
• Added unit tests for both existing and new CreateSessionAsync overloads

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Added new CreateSessionAsync overload accepting both contextId and taskId parameters
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs	Added two test methods to verify both CreateSessionAsync overloads create sessions with correct IDs

[westey-m]

@copilot, the unit tests are failing, please fix, and address the PR comments.

  Failed Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(taskId: "\r\n") [1 ms]
  Error Message:
   Assert.Throws() Failure: No exception was thrown
Expected: typeof(System.ArgumentException)
  Stack Trace:
     at Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(String taskId) in /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs:line 1240
--- End of stack trace from previous location ---
  Failed Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(taskId: null) [< 1 ms]
  Error Message:
   Assert.Throws() Failure: No exception was thrown
Expected: typeof(System.ArgumentException)
  Stack Trace:
     at Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(String taskId) in /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs:line 1240
--- End of stack trace from previous location ---
  Failed Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(taskId: " ") [< 1 ms]
  Error Message:
   Assert.Throws() Failure: No exception was thrown
Expected: typeof(System.ArgumentException)
  Stack Trace:
     at Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(String taskId) in /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs:line 1240
--- End of stack trace from previous location ---
  Failed Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(taskId: "\t") [< 1 ms]
  Error Message:
   Assert.Throws() Failure: No exception was thrown
Expected: typeof(System.ArgumentException)
  Stack Trace:
     at Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(String taskId) in /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs:line 1240
--- End of stack trace from previous location ---
  Failed Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(taskId: "") [< 1 ms]
  Error Message:
   Assert.Throws() Failure: No exception was thrown
Expected: typeof(System.ArgumentException)
  Stack Trace:
     at Microsoft.Agents.AI.A2A.UnitTests.A2AAgentTests.CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsArgumentExceptionAsync(String taskId) in /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs:line 1240
     ```

[Copilot]

@copilot, the unit tests are failing, please fix, and address the PR comments.

...

Fixed in commit 0f48857. Added parameter validation using Throw.IfNullOrWhitespace to both CreateSessionAsync overloads and updated tests to use ThrowsAnyAsync to handle both ArgumentNullException and ArgumentException. All 108 tests now passing.

[westey-m]

@copilot please address

[Copilot]

@copilot please address

Addressed in commit 74a6622.