Fix hosted agent samples Docker build failures due to experimental API warnings (#4641)

Add #pragma warning disable directives to suppress experimental API
diagnostics that cause build errors in Docker isolation (where repo-level
Directory.Build.props is not inherited):

- AgentWithHostedMCP: suppress MEAI001 (HostedMcpServerTool) and OPENAI001
  (GetResponsesClient)
- FoundrySingleAgent: suppress CA2252 (AIProjectClient preview features)
- FoundryMultiAgent: suppress CA2252 (AIProjectClient preview features)

Fixes #4365

dotnet/samples/05-end-to-end/HostedAgents/AgentWithHostedMCP/Program.cs

@@ -4,6 +4,9 @@
 // In this case the OpenAI responses service will invoke any MCP tools as required. MCP tools are not invoked by the Agent Framework.
 // The sample demonstrates how to use MCP tools with auto approval by setting ApprovalMode to NeverRequire.
 
+#pragma warning disable MEAI001 // HostedMcpServerTool, HostedMcpServerToolApprovalMode are experimental
+#pragma warning disable OPENAI001 // GetResponsesClient is experimental
+
 using Azure.AI.AgentServer.AgentFramework.Extensions;
 using Azure.AI.OpenAI;
 using Azure.Identity;

dotnet/samples/05-end-to-end/HostedAgents/FoundryMultiAgent/Program.cs

@@ -3,6 +3,8 @@
 // This sample demonstrates a multi-agent workflow with Writer and Reviewer agents
 // using Azure AI Foundry AIProjectClient and the Agent Framework WorkflowBuilder.
 
+#pragma warning disable CA2252 // AIProjectClient and Agents API require opting into preview features
+
 using Azure.AI.AgentServer.AgentFramework.Extensions;
 using Azure.AI.Projects;
 using Azure.Identity;

dotnet/samples/05-end-to-end/HostedAgents/FoundrySingleAgent/Program.cs

@@ -4,6 +4,8 @@
 // Uses Microsoft Agent Framework with Azure AI Foundry.
 // Ready for deployment to Foundry Hosted Agent service.
 
+#pragma warning disable CA2252 // AIProjectClient and Agents API require opting into preview features
+
 using System.ComponentModel;
 using System.Globalization;
 using System.Text;