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/Contracts/ProtocolModels.cs

@@ -177,6 +177,13 @@ public sealed class ResolveApprovalCommandDto : SidecarCommandEnvelope
     public string Decision { get; init; } = string.Empty;
 }
 
+public sealed class ResolveUserInputCommandDto : SidecarCommandEnvelope
+{
+    public string UserInputId { get; init; } = string.Empty;
+    public string Answer { get; init; } = string.Empty;
+    public bool WasFreeform { get; init; }
+}
+
 public sealed class RunTurnToolingConfigDto
 {
     public IReadOnlyList<RunTurnMcpServerConfigDto> McpServers { get; init; } = [];
@@ -290,6 +297,17 @@ public sealed class ApprovalRequestedEventDto : SidecarEventDto
     public PermissionDetailDto? PermissionDetail { get; init; }
 }
 
+public sealed class UserInputRequestedEventDto : SidecarEventDto
+{
+    public string SessionId { get; init; } = string.Empty;
+    public string UserInputId { get; init; } = string.Empty;
+    public string? AgentId { get; init; }
+    public string? AgentName { get; init; }
+    public string Question { get; init; } = string.Empty;
+    public IReadOnlyList<string>? Choices { get; init; }
+    public bool? AllowFreeform { get; init; }
+}
+
 public sealed class CommandErrorEventDto : SidecarEventDto
 {
     public string Message { get; init; } = string.Empty;