feat: add ask_user support to sidecar

Implement the Copilot SDK user input round-trip in the backend:
- add user-input-requested and resolve-user-input protocol DTOs
- add a CopilotUserInputCoordinator that mirrors approval flow with
  pending TaskCompletionSource state and explicit resolution
- wire SessionConfig.OnUserInputRequest through CopilotAgentBundle
  and CopilotWorkflowRunner
- extend SidecarProtocolHost to emit user input events, accept
  resolve-user-input commands, and filter ask_user from runtime
  approval tools
- add regression tests for the new coordinator and protocol flow

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

sidecar/src/Aryx.AgentHost/Services/ITurnWorkflowRunner.cs

@@ -9,9 +9,14 @@ public interface ITurnWorkflowRunner
         Func<TurnDeltaEventDto, Task> onDelta,
         Func<AgentActivityEventDto, Task> onActivity,
         Func<ApprovalRequestedEventDto, Task> onApproval,
+        Func<UserInputRequestedEventDto, Task> onUserInput,
         CancellationToken cancellationToken);
 
     Task ResolveApprovalAsync(
         ResolveApprovalCommandDto command,
         CancellationToken cancellationToken);
+
+    Task ResolveUserInputAsync(
+        ResolveUserInputCommandDto command,
+        CancellationToken cancellationToken);
 }