Run subagent request handler test over stdio

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

Author: stephentoub

dotnet/test/E2E/SubagentHooksE2ETests.cs

@@ -26,6 +26,7 @@ public async Task Should_Invoke_PreToolUse_And_PostToolUse_Hooks_For_Sub_Agent_T
         env["COPILOT_EXP_COPILOT_CLI_SESSION_BASED_SUBAGENTS"] = "true";
         var client = Ctx.CreateClient(new CopilotClientOptions
         {
+            Connection = RuntimeConnection.ForStdio(),
             RequestHandler = requestHandler
         }, environment: env);