.Net: Add sample showing how to use web and file search

[markwallace-microsoft]

Motivation and Context

Partial fix for #12562

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull Request Overview

This PR adds samples demonstrating how to utilize web and file search capabilities with the OpenAIResponseAgent, along with updates to support reasoning models and new agent invoke options.

• Updated BaseResponsesAgentTest to use ChatModelId and added FileClient and VectorStoreClient.
• Changed invoke option types in OpenAIResponseAgent to OpenAIResponseAgentInvokeOptions.
• Added new sample files demonstrating tool usage for web search, file search, and reasoning with summaries.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/InternalUtilities/samples/AgentUtilities/BaseResponsesAgentTest.cs	Updated client instantiation, changed model identifier, and added file and vector store clients; EnableLogging default has changed.
dotnet/src/Agents/OpenAI/OpenAIResponseAgentInvokeOptions.cs	Added support to propagate ResponseCreationOptions in constructor overloads.
dotnet/src/Agents/OpenAI/OpenAIResponseAgent.cs	Updated invoke options type from OpenAIAssistantAgentInvokeOptions to OpenAIResponseAgentInvokeOptions.
dotnet/src/Agents/OpenAI/Extensions/OpenAIResponseExtensions.cs	Extended response conversion for ReasoningResponseItem and added helper conversion for summary texts.
dotnet/samples/GettingStartedWithAgents/OpenAIResponse/Step04_OpenAIResponseAgent_Tools.cs	New sample demonstrating usage of web search and file search tools.
dotnet/samples/GettingStartedWithAgents/OpenAIResponse/Step03_OpenAIResponseAgent_ReasoningModel.cs	Added test for using a reasoning model with summaries, including multi-line prompt usage.

[markwallace-microsoft]

One unrelated integration tests failure

[xUnit.net 00:01:46.63]     SemanticKernel.IntegrationTests.Connectors.AzureOpenAI.AzureOpenAIChatClientRequiredFunctionChoiceBehaviorTests.SpecifiedInPromptInstructsConnectorToInvokeKernelFunctionAutomaticallyAsync [FAIL]
[xUnit.net 00:01:46.63]       Microsoft.SemanticKernel.KernelFunctionCanceledException : The invocation of function 'Function_6310afcda1474806a53c33cdbc83035e' was canceled.
[xUnit.net 00:01:46.63]       ---- System.Threading.Tasks.TaskCanceledException : The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing.
[xUnit.net 00:01:46.63]       -------- System.TimeoutException : The operation was canceled.
[xUnit.net 00:01:46.63]       ------------ System.Threading.Tasks.TaskCanceledException : The operation was canceled.
[xUnit.net 00:01:46.63]       ---------------- System.IO.IOException : Unable to read data from the transport connection: Operation canceled.
[xUnit.net 00:01:46.63]       -------------------- System.Net.Sockets.SocketException : Operation canceled