Add SetVolumeTool (validates 0..100, delegates to Pi via IDeviceChannel)

backend.tests/SetVolumeToolTests.cs

@@ -0,0 +1,52 @@
+using System.Text.Json;
+using backend.Tools;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class SetVolumeToolTests
+{
+    [Fact]
+    public async Task Forwards_call_to_pi_and_returns_its_result()
+    {
+        var fake = new RecordingChannel(
+            JsonDocument.Parse("""{"ok":true}""").RootElement);
+        var tool = new SetVolumeTool();
+        var ctx = new DeviceContext(Guid.NewGuid(), Guid.NewGuid(), fake);
+
+        var args = JsonDocument.Parse("""{"level":42}""").RootElement;
+        var res = await tool.ExecuteAsync(new ToolInvocation("c1", args), ctx, default);
+
+        fake.LastName.Should().Be("set_volume");
+        fake.LastArgs.GetProperty("level").GetInt32().Should().Be(42);
+        res.Output.GetProperty("ok").GetBoolean().Should().BeTrue();
+    }
+
+    [Fact]
+    public async Task Rejects_out_of_range_level_without_calling_pi()
+    {
+        var fake = new RecordingChannel(JsonDocument.Parse("""{"ok":true}""").RootElement);
+        var tool = new SetVolumeTool();
+        var ctx = new DeviceContext(Guid.NewGuid(), Guid.NewGuid(), fake);
+
+        var args = JsonDocument.Parse("""{"level":150}""").RootElement;
+        var res = await tool.ExecuteAsync(new ToolInvocation("c1", args), ctx, default);
+
+        fake.LastName.Should().BeNull();
+        res.Output.GetProperty("ok").GetBoolean().Should().BeFalse();
+        res.Output.GetProperty("error").GetString().Should().Contain("0..100");
+    }
+
+    private class RecordingChannel(JsonElement reply) : IDeviceChannel
+    {
+        public string? LastName { get; private set; }
+        public JsonElement LastArgs { get; private set; }
+        public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
+        {
+            LastName = name;
+            LastArgs = args;
+            return Task.FromResult(reply);
+        }
+    }
+}

backend/Tools/SetVolumeTool.cs

@@ -0,0 +1,34 @@
+using System.Text.Json;
+
+namespace backend.Tools;
+
+public class SetVolumeTool : ITool
+{
+    public string Name => "set_volume";
+    public string Description =>
+        "Sets the assistant's audio output volume on the device. Level is 0..100.";
+    public bool RunsDuringResponse => true;
+
+    public JsonElement ParameterSchema { get; } = JsonDocument.Parse(
+        """
+        {
+          "type":"object",
+          "properties":{"level":{"type":"integer","minimum":0,"maximum":100}},
+          "required":["level"]
+        }
+        """).RootElement;
+
+    public async Task<ToolResult> ExecuteAsync(ToolInvocation invocation, DeviceContext device, CancellationToken ct)
+    {
+        if (!invocation.Arguments.TryGetProperty("level", out var lvlEl)
+            || !lvlEl.TryGetInt32(out var level)
+            || level < 0 || level > 100)
+        {
+            var err = JsonDocument.Parse("""{"ok":false,"error":"level must be an integer in 0..100"}""").RootElement;
+            return new ToolResult(err);
+        }
+
+        var piReply = await device.Channel.CallPiToolAsync(Name, invocation.Arguments, ct);
+        return new ToolResult(piReply);
+    }
+}