.NET: [BREAKING] Remove UserInputRequests property

[westey-m]

Motivation and Context

Updating based on review feedback: #2541

Description

• Removing the UserInputRequests property from AgentResponse and AgentResponseUpdate, since users often need to filter further anyway, and there isn't a clear consensus as to what constitutes a user input request. Can add back later if there is more consensus.

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

This pull request removes the UserInputRequests convenience property from AgentResponse and AgentResponseUpdate classes, requiring users to manually filter content for user input requests. This breaking change addresses feedback from issue #2541, where there wasn't clear consensus on what constitutes a user input request.

Changes:

• Removed UserInputRequests property from AgentResponse and AgentResponseUpdate classes
• Removed unused System.Linq imports from abstraction files
• Updated all sample code to use explicit filtering with SelectMany(m => m.Contents).OfType<T>()
• Removed associated unit tests for the deleted properties

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponse.cs	Removed UserInputRequests property and unused System.Linq import
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponseUpdate.cs	Removed UserInputRequests property and unused System.Linq import
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseTests.cs	Removed unit tests for deleted UserInputRequests property
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateTests.cs	Removed unit tests for deleted UserInputRequests property
dotnet/samples/M365Agent/AFAgentApplication.cs	Updated to use explicit filtering for function approval requests with updated comments
dotnet/samples/GettingStarted/ModelContextProtocol/ResponseAgent_Hosted_MCP/Program.cs	Updated to filter MCP tool approval requests explicitly, using explicit types
dotnet/samples/GettingStarted/ModelContextProtocol/FoundryAgent_Hosted_MCP/Program.cs	Updated to filter MCP tool approval requests explicitly, using explicit types
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step12_Middleware/Program.cs	Updated middleware to filter function approval requests explicitly
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step04_UsingFunctionToolsWithApprovals/Program.cs	Updated to filter function approval requests explicitly, using explicit types
dotnet/samples/GettingStarted/Agents/Agent_Step14_Middleware/Program.cs	Updated middleware to filter function approval requests explicitly
dotnet/samples/GettingStarted/Agents/Agent_Step04_UsingFunctionToolsWithApprovals/Program.cs	Updated to filter function approval requests explicitly, using explicit types