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/CopilotWorkflowRunner.cs

@@ -8,6 +8,7 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
 {
     private readonly PatternValidator _patternValidator;
     private readonly CopilotApprovalCoordinator _approvalCoordinator = new();
+    private readonly CopilotUserInputCoordinator _userInputCoordinator = new();
 
     public CopilotWorkflowRunner(PatternValidator patternValidator)
     {
@@ -19,6 +20,7 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
         Func<TurnDeltaEventDto, Task> onDelta,
         Func<AgentActivityEventDto, Task> onActivity,
         Func<ApprovalRequestedEventDto, Task> onApproval,
+        Func<UserInputRequestedEventDto, Task> onUserInput,
         CancellationToken cancellationToken)
     {
         PatternValidationIssueDto? validationError = _patternValidator.Validate(command.Pattern).FirstOrDefault();
@@ -38,6 +40,13 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
                 state.ToolNamesByCallId,
                 onApproval,
                 cancellationToken),
+            (agent, request, invocation) => _userInputCoordinator.RequestUserInputAsync(
+                command,
+                agent,
+                request,
+                invocation,
+                onUserInput,
+                cancellationToken),
             (agent, sessionEvent) => state.ObserveSessionEvent(agent, sessionEvent),
             cancellationToken);
         Workflow workflow = bundle.BuildWorkflow(command.Pattern);
@@ -66,6 +75,13 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
         return _approvalCoordinator.ResolveApprovalAsync(command, cancellationToken);
     }
 
+    public Task ResolveUserInputAsync(
+        ResolveUserInputCommandDto command,
+        CancellationToken cancellationToken)
+    {
+        return _userInputCoordinator.ResolveUserInputAsync(command, cancellationToken);
+    }
+
     private static async Task<bool> HandleWorkflowEventAsync(
         RunTurnCommandDto command,
         WorkflowEvent evt,