test: add xUnit test coverage for API controllers and client services

Create ChatAgent.Api.Tests and ChatAgent.Client.Tests projects with xUnit
and Moq. Test HealthController (200 + valid response), ChatController
(SSE streaming with mocked upstream, error handling), and ChatApiClient
(delta parsing, error events, health endpoint). 6 tests, all passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/ChatAgent.Client.Tests/ChatApiClientTests.cs

@@ -0,0 +1,141 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text;
+using ChatAgent.Client.Services;
+using ChatAgent.Shared.Models;
+
+namespace ChatAgent.Client.Tests;
+
+public class ChatApiClientTests
+{
+    [Fact]
+    public async Task SendChatStreamingAsync_ParsesTextDeltas_InOrder()
+    {
+        // Arrange: build a canned SSE response with two text deltas
+        var sse = BuildSSE(
+            ("text", "Hello"),
+            ("text", " world")
+        );
+
+        var handler = new FakeHttpMessageHandler(new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(sse, Encoding.UTF8, "text/event-stream")
+        });
+
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new ChatApiClient(httpClient);
+
+        var request = new ChatRequest
+        {
+            Messages = new List<ChatMessage>
+            {
+                new() { Role = "user", Content = "Hi" }
+            }
+        };
+
+        // Act
+        var deltas = new List<string>();
+        await foreach (var delta in client.SendChatStreamingAsync(request))
+        {
+            deltas.Add(delta);
+        }
+
+        // Assert
+        Assert.Equal(2, deltas.Count);
+        Assert.Equal("Hello", deltas[0]);
+        Assert.Equal(" world", deltas[1]);
+    }
+
+    [Fact]
+    public async Task SendChatStreamingAsync_ThrowsOnErrorEvent()
+    {
+        // Arrange
+        var sse = "data: {\"error\":\"Something went wrong\"}\n\n" +
+                  "data: [DONE]\n\n";
+
+        var handler = new FakeHttpMessageHandler(new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(sse, Encoding.UTF8, "text/event-stream")
+        });
+
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new ChatApiClient(httpClient);
+
+        var request = new ChatRequest
+        {
+            Messages = new List<ChatMessage>
+            {
+                new() { Role = "user", Content = "Hi" }
+            }
+        };
+
+        // Act & Assert
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(async () =>
+        {
+            await foreach (var _ in client.SendChatStreamingAsync(request))
+            {
+            }
+        });
+
+        Assert.Contains("Something went wrong", ex.Message);
+    }
+
+    [Fact]
+    public async Task GetHealthAsync_ReturnsHealthResponse()
+    {
+        // Arrange
+        var healthJson = "{\"status\":\"healthy\",\"timestamp\":\"2026-04-04T12:00:00Z\"}";
+        var handler = new FakeHttpMessageHandler(new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(healthJson, Encoding.UTF8, "application/json")
+        });
+
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new ChatApiClient(httpClient);
+
+        // Act
+        var result = await client.GetHealthAsync();
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("healthy", result.Status);
+    }
+
+    /// <summary>
+    /// Builds simplified SSE content from (type, value) pairs.
+    /// Each pair becomes: data: {"text":"value"}\n\n
+    /// Ends with data: [DONE]\n\n
+    /// </summary>
+    private static string BuildSSE(params (string key, string value)[] events)
+    {
+        var sb = new StringBuilder();
+        foreach (var (key, value) in events)
+        {
+            sb.AppendLine($"data: {{\"{key}\":\"{value}\"}}");
+            sb.AppendLine();
+        }
+        sb.AppendLine("data: [DONE]");
+        sb.AppendLine();
+        return sb.ToString();
+    }
+}
+
+/// <summary>
+/// Simple fake HttpMessageHandler for unit testing HttpClient-based services.
+/// Returns a canned response for any request.
+/// </summary>
+public class FakeHttpMessageHandler : HttpMessageHandler
+{
+    private readonly HttpResponseMessage _response;
+
+    public FakeHttpMessageHandler(HttpResponseMessage response)
+    {
+        _response = response;
+    }
+
+    protected override Task<HttpResponseMessage> SendAsync(
+        HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        return Task.FromResult(_response);
+    }
+}