.NET: Add MCP long-running task support for MCP client tools

[peibekwe]

Description

Adds Microsoft Agent Framework client-side support for the MCP tasks extension.

A new Microsoft.Agents.AI.Mcp (alpha) package introduces McpClient.ListAgentToolsWithTaskSupportAsync, which wraps any MCP tool advertising task support with a task-aware AIFunction. The wrapper transparently drives the lifecycle inside the agent's function-calling loop.

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.

[Copilot]

Pull request overview

Adds a new .NET (alpha) Microsoft.Agents.AI.Mcp package that enables Microsoft Agent Framework agents to transparently invoke MCP tools using the MCP long-running tasks extension (SEP-2663), including polling to completion and best-effort remote cancellation.

Changes:

• Introduces McpClientTaskExtensions.ListAgentToolsWithTaskSupportAsync(...) to wrap task-capable MCP tools (Optional/Required) in a task-aware AIFunction.
• Implements TaskAwareMcpClientAIFunction and McpTaskOptions to drive tools/call (task-augmented) → tasks/get polling → tasks/result, with fallback to non-task invocation when needed.
• Adds unit tests + an end-to-end sample demonstrating transparent long-running task handling, and wires the sample into sample verification + solution.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Mcp/Microsoft.Agents.AI.Mcp.csproj	New alpha MCP integration package project definition and dependencies.
dotnet/src/Microsoft.Agents.AI.Mcp/McpClientTaskExtensions.cs	Adds extension method to list tools and wrap task-capable ones for agent usage.
dotnet/src/Microsoft.Agents.AI.Mcp/McpTaskOptions.cs	Defines configurable task lifecycle options (TTL hint + remote-cancel behavior).
dotnet/src/Microsoft.Agents.AI.Mcp/TaskAwareMcpClientAIFunction.cs	Implements the task-aware wrapper driving call/poll/result + cancellation handling.
dotnet/tests/Microsoft.Agents.AI.Mcp.UnitTests/Microsoft.Agents.AI.Mcp.UnitTests.csproj	Adds new unit test project for the MCP task-aware client functionality.
dotnet/tests/Microsoft.Agents.AI.Mcp.UnitTests/InMemoryMcpServerFixture.cs	In-proc server/client fixture for testing tasks behavior without sockets/processes.
dotnet/tests/Microsoft.Agents.AI.Mcp.UnitTests/TestTools.cs	Test helper to create MCP tools with specific task support metadata.
dotnet/tests/Microsoft.Agents.AI.Mcp.UnitTests/ListAgentToolsWithTaskSupportTests.cs	Validates wrapping behavior + null client argument validation.
dotnet/tests/Microsoft.Agents.AI.Mcp.UnitTests/TaskAwareMcpClientAIFunctionTests.cs	Validates happy paths, TTL propagation, cancellation, and failure behavior.
dotnet/tests/Microsoft.Agents.AI.Mcp.UnitTests/McpTaskOptionsTests.cs	Verifies option defaults.
dotnet/samples/02-agents/ModelContextProtocol/README.md	Adds the new long-running task client sample to the MCP samples index.
dotnet/samples/02-agents/ModelContextProtocol/Agent_MCP_LongRunningTask_Client/README.md	New sample documentation describing task support behavior and usage.
dotnet/samples/02-agents/ModelContextProtocol/Agent_MCP_LongRunningTask_Client/Program.cs	New sample demonstrating stdio MCP server + client wrapper + Run/RunStreaming.
dotnet/samples/02-agents/ModelContextProtocol/Agent_MCP_LongRunningTask_Client/Agent_MCP_LongRunningTask_Client.csproj	New sample project wiring required dependencies and project references.
dotnet/eng/verify-samples/AgentsSamples.cs	Registers the new sample for verification.
dotnet/agent-framework-dotnet.slnx	Adds new package, test project, and sample to the solution.