Add EndSessionTool (no params, returns ok; relay closes on this tool name)

backend.tests/EndSessionToolTests.cs

@@ -0,0 +1,35 @@
+using System.Text.Json;
+using backend.Tools;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class EndSessionToolTests
+{
+    [Fact]
+    public async Task Returns_ok_true()
+    {
+        var tool = new EndSessionTool();
+        var ctx = new DeviceContext(Guid.NewGuid(), Guid.NewGuid(), new NoopChannel());
+
+        var res = await tool.ExecuteAsync(
+            new ToolInvocation("call_1", JsonDocument.Parse("{}").RootElement), ctx, default);
+
+        res.Output.GetProperty("ok").GetBoolean().Should().BeTrue();
+    }
+
+    [Fact]
+    public void Name_and_runs_during_response_match_spec()
+    {
+        var tool = new EndSessionTool();
+        tool.Name.Should().Be("end_session");
+        tool.RunsDuringResponse.Should().BeFalse();
+    }
+
+    private class NoopChannel : IDeviceChannel
+    {
+        public Task<JsonElement> CallPiToolAsync(string n, JsonElement a, CancellationToken ct)
+            => throw new NotSupportedException();
+    }
+}

backend/Tools/EndSessionTool.cs

@@ -0,0 +1,20 @@
+using System.Text.Json;
+
+namespace backend.Tools;
+
+public class EndSessionTool : ITool
+{
+    public string Name => "end_session";
+    public string Description =>
+        "Call this when the user has said goodbye or otherwise indicates the conversation is over.";
+    public bool RunsDuringResponse => false;
+
+    public JsonElement ParameterSchema { get; } =
+        JsonDocument.Parse("""{"type":"object","properties":{},"required":[]}""").RootElement;
+
+    public Task<ToolResult> ExecuteAsync(ToolInvocation invocation, DeviceContext device, CancellationToken ct)
+    {
+        var output = JsonDocument.Parse("""{"ok":true}""").RootElement;
+        return Task.FromResult(new ToolResult(output));
+    }
+}