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/tests/Aryx.AgentHost.Tests/CopilotUserInputCoordinatorTests.cs

@@ -0,0 +1,109 @@
+using Aryx.AgentHost.Contracts;
+using Aryx.AgentHost.Services;
+using GitHub.Copilot.SDK;
+
+namespace Aryx.AgentHost.Tests;
+
+public sealed class CopilotUserInputCoordinatorTests
+{
+    [Fact]
+    public async Task RequestUserInputAsync_RaisesUserInputEventAndCompletesAfterResolution()
+    {
+        CopilotUserInputCoordinator coordinator = new();
+        UserInputRequestedEventDto? observedEvent = null;
+        RunTurnCommandDto command = CreateUserInputCommand();
+
+        Task<UserInputResponse> pending = coordinator.RequestUserInputAsync(
+            command,
+            command.Pattern.Agents[0],
+            new UserInputRequest
+            {
+                Question = "How should I proceed?",
+                Choices = ["Continue", "Stop"],
+                AllowFreeform = true,
+            },
+            new UserInputInvocation
+            {
+                SessionId = "copilot-session-1",
+            },
+            userInputEvent =>
+            {
+                observedEvent = userInputEvent;
+                return Task.CompletedTask;
+            },
+            CancellationToken.None);
+
+        Assert.False(pending.IsCompleted);
+        Assert.NotNull(observedEvent);
+        Assert.Equal("user-input-requested", observedEvent!.Type);
+        Assert.Equal("turn-1", observedEvent.RequestId);
+        Assert.Equal("session-1", observedEvent.SessionId);
+        Assert.Equal("agent-1", observedEvent.AgentId);
+        Assert.Equal("Primary", observedEvent.AgentName);
+        Assert.Equal("How should I proceed?", observedEvent.Question);
+        Assert.Equal(["Continue", "Stop"], observedEvent.Choices);
+        Assert.True(observedEvent.AllowFreeform);
+
+        await coordinator.ResolveUserInputAsync(
+            new ResolveUserInputCommandDto
+            {
+                UserInputId = observedEvent.UserInputId,
+                Answer = "Continue",
+                WasFreeform = false,
+            },
+            CancellationToken.None);
+
+        UserInputResponse response = await pending;
+        Assert.Equal("Continue", response.Answer);
+        Assert.False(response.WasFreeform);
+    }
+
+    [Fact]
+    public async Task ResolveUserInputAsync_RejectsUnknownUserInputIds()
+    {
+        CopilotUserInputCoordinator coordinator = new();
+
+        InvalidOperationException error = await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            coordinator.ResolveUserInputAsync(
+                new ResolveUserInputCommandDto
+                {
+                    UserInputId = "user-input-missing",
+                    Answer = "Continue",
+                    WasFreeform = false,
+                },
+                CancellationToken.None));
+
+        Assert.Contains("is not pending", error.Message);
+    }
+
+    private static PatternAgentDefinitionDto CreateAgent(string id, string name)
+    {
+        return new PatternAgentDefinitionDto
+        {
+            Id = id,
+            Name = name,
+            Model = "gpt-5.4",
+            Instructions = "Help with the request.",
+        };
+    }
+
+    private static RunTurnCommandDto CreateUserInputCommand()
+    {
+        return new RunTurnCommandDto
+        {
+            RequestId = "turn-1",
+            SessionId = "session-1",
+            Pattern = new PatternDefinitionDto
+            {
+                Id = "pattern-1",
+                Name = "User Input Pattern",
+                Mode = "single",
+                Availability = "available",
+                Agents =
+                [
+                    CreateAgent("agent-1", "Primary"),
+                ],
+            },
+        };
+    }
+}