.NET: Remove Foundry Toolbox server-side tools support (#5753)

* .NET: Remove Foundry Toolbox server-side tools support

Mirrors the Python cleanup in microsoft/agent-framework#5671. Passing
toolbox tools as server-side Responses tools is not the experience we
want to support; the hosted-agent MCP toolbox path (HostedMcpToolboxAITool
+ FoundryToolboxService) remains the supported way to consume Foundry
Toolboxes.

Removed:
- FoundryToolbox static class (GetToolboxVersionAsync / GetToolsAsync /
  ToAITools / SanitizeAndConvert)
- AIProjectClient.GetToolboxToolsAsync extension
- Agent_Step25_ToolboxServerSideTools sample (+ slnx entry)
- FoundryToolboxTests, TestDataUtil, HttpHandlerAssert, and the toolbox
  JSON fixtures only those tests referenced
- ToolboxHostedAgentTests and ToolboxHostedAgentFixture; the "toolbox"
  switch arm + CreateToolboxAgent helper in TestContainer; matching
  README scenario row and bootstrap script entry

Kept (MCP path, unchanged):
- HostedMcpToolboxAITool, FoundryAITool.CreateHostedMcpToolbox,
  FoundryAIToolExtensions.CreateHostedMcpToolbox(ToolboxRecord/Version)
- FoundryToolboxService, AddFoundryToolboxes, marker injection in
  AgentFrameworkResponseHandler, InputConverter.ReadMcpToolboxMarkers
- Hosted-Toolbox sample, McpToolbox* tests, FoundryToolboxServiceTests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* .NET: Add Foundry Toolbox MCP sample (Agent_Step25_FoundryToolboxMcp)

Adds a non-hosted-agent equivalent of the Python foundry_chat_client_with_toolbox.py sample. The agent connects to a Foundry Toolbox's MCP endpoint via Streamable HTTP, injects a fresh Azure AI bearer token on every request, and discovers the toolbox's tools at runtime via McpClient.ListToolsAsync.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* .NET: Tighten Agent_Step25_FoundryToolboxMcp README/Program comments

Drop 'non-hosted agent' framing from README (this sample isn't related to hosted agents) and remove narrative comparison to server-side tools from the Program.cs header comment.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Drop python sample reference from Agent_Step25 README

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Drop incorrect .NET 10 prereq from Agent_Step25 README

Toolboxes don't require .NET 10 (Microsoft.Agents.AI.Foundry targets net8.0+); the parent AgentsWithFoundry README already lists the sample SDK prereq.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix Toolsets api-version in Agent_Step25 example endpoint

Use 2025-05-01-preview to match FoundryToolboxOptions.ApiVersion. The placeholder 'v1' is not accepted by the Toolsets endpoint.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: alliscode <bentho@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/tests/Foundry.Hosting.IntegrationTests.TestContainer/Program.cs

@@ -32,7 +32,6 @@ AIAgent agent = scenario switch
     "happy-path" => CreateHappyPathAgent(projectClient, deployment),
     "tool-calling" => CreateToolCallingAgent(projectClient, deployment),
     "tool-calling-approval" => CreateToolCallingApprovalAgent(projectClient, deployment),
-    "toolbox" => CreateToolboxAgent(projectClient, deployment),
     "mcp-toolbox" => CreateMcpToolboxAgent(projectClient, deployment),
     "custom-storage" => CreateCustomStorageAgent(projectClient, deployment),
     "azure-search-rag" => CreateAzureSearchRagAgent(projectClient, deployment),
@@ -84,17 +83,6 @@ static AIAgent CreateToolCallingApprovalAgent(AIProjectClient client, string dep
             AIFunctionFactory.Create(SendEmail)
         ]);
 
-static AIAgent CreateToolboxAgent(AIProjectClient client, string deployment) =>
-    // TODO: wire Foundry toolbox host once API surface is finalized for hosted agents.
-    client.AsAIAgent(
-        model: deployment,
-        instructions: "You are a toolbox enabled assistant. Use GetEnvironmentName when asked.",
-        name: "toolbox-agent",
-        description: "Toolbox test agent (placeholder).",
-        tools: [
-            AIFunctionFactory.Create(GetEnvironmentName)
-        ]);
-
 static AIAgent CreateMcpToolboxAgent(AIProjectClient client, string deployment) =>
     // TODO: wire MCP toolbox client to https://learn.microsoft.com/api/mcp.
     client.AsAIAgent(
@@ -203,9 +191,6 @@ static string SendEmail(
     [Description("Email subject")] string subject) =>
     $"Email sent to {to} with subject '{subject}'.";
 
-[Description("Returns the deployment environment name.")]
-static string GetEnvironmentName() => "integration-test";
-
 // session-files tools: resolve paths against $HOME (the per-session sandbox volume).
 [Description("Get the absolute path of the session home directory ($HOME).")]
 static string GetHomeDirectory() => SessionHome();