chore: import upstream snapshot with attribution

dotnet/samples/02-agents/AgentProviders/foundry/Agent_Step16_FileSearch/Agent_Step16_FileSearch.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFrameworks>net10.0</TargetFrameworks>
+
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Azure.Identity" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\..\..\src\Microsoft.Agents.AI.Foundry\Microsoft.Agents.AI.Foundry.csproj" />
+  </ItemGroup>
+
+</Project>

dotnet/samples/02-agents/AgentProviders/foundry/Agent_Step16_FileSearch/Program.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample shows how to use File Search Tool with a ChatClientAgent.
+
+using Azure.AI.Projects;
+using Azure.Identity;
+using Microsoft.Agents.AI;
+using Microsoft.Extensions.AI;
+using OpenAI.Assistants;
+using OpenAI.Files;
+
+string endpoint = Environment.GetEnvironmentVariable("FOUNDRY_PROJECT_ENDPOINT") ?? throw new InvalidOperationException("FOUNDRY_PROJECT_ENDPOINT is not set.");
+string deploymentName = Environment.GetEnvironmentVariable("FOUNDRY_MODEL") ?? "gpt-5.4-mini";
+
+const string AgentInstructions = "You are a helpful assistant that can search through uploaded files to answer questions.";
+
+// We need the AIProjectClient to upload files and create vector stores.
+// WARNING: DefaultAzureCredential is convenient for development but requires careful consideration in production.
+// In production, consider using a specific credential (e.g., ManagedIdentityCredential) to avoid
+// latency issues, unintended credential probing, and potential security risks from fallback mechanisms.
+AIProjectClient aiProjectClient = new(new Uri(endpoint), new DefaultAzureCredential());
+var projectOpenAIClient = aiProjectClient.GetProjectOpenAIClient();
+var filesClient = projectOpenAIClient.GetProjectFilesClient();
+var vectorStoresClient = projectOpenAIClient.GetProjectVectorStoresClient();
+
+// 1. Create a temp file with test content and upload it.
+string searchFilePath = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName() + "_lookup.txt");
+File.WriteAllText(
+    path: searchFilePath,
+    contents: """
+        Employee Directory:
+        - Alice Johnson, 28 years old, Software Engineer, Engineering Department
+        - Bob Smith, 35 years old, Sales Manager, Sales Department
+        - Carol Williams, 42 years old, HR Director, Human Resources Department
+        - David Brown, 31 years old, Customer Support Lead, Support Department
+        """
+);
+
+Console.WriteLine($"Uploading file: {searchFilePath}");
+OpenAIFile uploadedFile = filesClient.UploadFile(
+    filePath: searchFilePath,
+    purpose: FileUploadPurpose.Assistants
+);
+Console.WriteLine($"Uploaded file, file ID: {uploadedFile.Id}");
+
+// 2. Create a vector store with the uploaded file.
+var vectorStoreResult = await vectorStoresClient.CreateVectorStoreAsync(
+    options: new() { FileIds = { uploadedFile.Id }, Name = "EmployeeDirectory_VectorStore" }
+);
+string vectorStoreId = vectorStoreResult.Value.Id;
+Console.WriteLine($"Created vector store, vector store ID: {vectorStoreId}");
+
+// Create a AIAgent with HostedFileSearchTool.
+AIAgent agent = aiProjectClient.AsAIAgent(deploymentName,
+    instructions: AgentInstructions,
+    name: "FileSearchAgent-RAPI",
+    tools: [new HostedFileSearchTool() { Inputs = [new HostedVectorStoreContent(vectorStoreId)] }]);
+
+// Run the agent
+Console.WriteLine("\n--- Running File Search Agent ---");
+AgentResponse response = await agent.RunAsync("Who is the youngest employee?");
+Console.WriteLine($"Response: {response}");
+
+// Getting any file citation annotations generated by the tool
+foreach (AIAnnotation annotation in response.Messages.SelectMany(m => m.Contents).SelectMany(c => c.Annotations ?? []))
+{
+    if (annotation.RawRepresentation is TextAnnotationUpdate citationAnnotation)
+    {
+        Console.WriteLine($$"""
+            File Citation:
+              File Id: {{citationAnnotation.OutputFileId}}
+              Text to Replace: {{citationAnnotation.TextToReplace}}
+            """);
+    }
+}
+
+// Cleanup file resources.
+Console.WriteLine("\n--- Cleanup ---");
+await vectorStoresClient.DeleteVectorStoreAsync(vectorStoreId);
+await filesClient.DeleteFileAsync(uploadedFile.Id);
+File.Delete(searchFilePath);
+Console.WriteLine("Cleanup completed successfully.");

dotnet/samples/02-agents/AgentProviders/foundry/Agent_Step16_FileSearch/README.md

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+# File Search with the Responses API
+
+This sample shows how to use the File Search tool with a `ChatClientAgent` using the Responses API directly.
+
+## What this sample demonstrates
+
+- Uploading files and creating vector stores via `AIProjectClient`
+- Using `HostedFileSearchTool` with `ChatClientAgent`
+- Handling file citation annotations in agent responses
+- Cleaning up file resources after use
+
+## Prerequisites
+
+- .NET 10 SDK or later
+- Microsoft Foundry service endpoint and deployment configured
+- An authenticated Azure identity (for example, sign in with `az login`)
+
+Set the following environment variables:
+
+```powershell
+$env:FOUNDRY_PROJECT_ENDPOINT="https://your-foundry-service.services.ai.azure.com/api/projects/your-foundry-project"
+$env:FOUNDRY_MODEL="gpt-5.4-mini"
+```
+
+## Run the sample
+
+```powershell
+dotnet run
+```
+