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.Api.Tests/ChatControllerTests.cs

@@ -0,0 +1,158 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text;
+using System.Text.Json;
+using ChatAgent.Shared.Models;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ChatAgent.Api.Tests;
+
+public class ChatControllerTests : IClassFixture<WebApplicationFactory<Program>>
+{
+    private readonly WebApplicationFactory<Program> _factory;
+
+    public ChatControllerTests(WebApplicationFactory<Program> factory)
+    {
+        _factory = factory;
+    }
+
+    [Fact]
+    public async Task PostChat_StreamsTextDeltas_AndDone()
+    {
+        // Arrange: mock the upstream Responses API with canned SSE events
+        var sseContent = BuildSSE(
+            ("response.created", null),
+            ("response.output_text.delta", "Hello"),
+            ("response.output_text.delta", " world"),
+            ("response.completed", null)
+        );
+
+        var client = CreateClientWithMockedUpstream(sseContent);
+
+        var request = new ChatRequest
+        {
+            Messages = new List<ChatMessage>
+            {
+                new() { Role = "user", Content = "Hi" }
+            }
+        };
+
+        // Act
+        var response = await client.PostAsJsonAsync("/api/chat", request);
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal("text/event-stream", response.Content.Headers.ContentType?.MediaType);
+
+        var body = await response.Content.ReadAsStringAsync();
+        var events = ParseSSEData(body);
+
+        // Assert: should contain text deltas and [DONE]
+        Assert.Contains(events, e => e.Contains("\"text\":\"Hello\""));
+        Assert.Contains(events, e => e.Contains("\"text\":\" world\""));
+        Assert.Contains(events, e => e == "[DONE]");
+    }
+
+    [Fact]
+    public async Task PostChat_HandlesUpstreamError_ReturnsErrorEvent()
+    {
+        // Arrange: upstream returns 500
+        var mockHandler = new MockHttpMessageHandler(
+            new HttpResponseMessage(HttpStatusCode.InternalServerError)
+            {
+                Content = new StringContent("Internal Server Error")
+            });
+
+        var client = CreateClientWithHandler(mockHandler);
+
+        var request = new ChatRequest
+        {
+            Messages = new List<ChatMessage>
+            {
+                new() { Role = "user", Content = "Hi" }
+            }
+        };
+
+        // Act
+        var response = await client.PostAsJsonAsync("/api/chat", request);
+        var body = await response.Content.ReadAsStringAsync();
+        var events = ParseSSEData(body);
+
+        // Assert: should contain error and [DONE]
+        Assert.Contains(events, e => e.Contains("\"error\""));
+        Assert.Contains(events, e => e == "[DONE]");
+    }
+
+    private HttpClient CreateClientWithMockedUpstream(string sseContent)
+    {
+        var mockHandler = new MockHttpMessageHandler(
+            new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent(sseContent, Encoding.UTF8, "text/event-stream")
+            });
+
+        return CreateClientWithHandler(mockHandler);
+    }
+
+    private HttpClient CreateClientWithHandler(MockHttpMessageHandler handler)
+    {
+        return _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                // Remove existing IHttpClientFactory registrations for "ResponsesApi"
+                // and replace with our mock
+                services.AddHttpClient("ResponsesApi")
+                    .ConfigurePrimaryHttpMessageHandler(() => handler);
+            });
+        }).CreateClient();
+    }
+
+    /// <summary>
+    /// Builds a fake SSE stream mimicking the Responses API format.
+    /// </summary>
+    private static string BuildSSE(params (string type, string? delta)[] events)
+    {
+        var sb = new StringBuilder();
+        foreach (var (type, delta) in events)
+        {
+            var data = delta != null
+                ? $"{{\"type\":\"{type}\",\"delta\":\"{delta}\"}}"
+                : $"{{\"type\":\"{type}\"}}";
+            sb.AppendLine($"event: {type}");
+            sb.AppendLine($"data: {data}");
+            sb.AppendLine();
+        }
+        return sb.ToString();
+    }
+
+    /// <summary>
+    /// Extracts the data payloads from SSE-formatted text.
+    /// </summary>
+    private static List<string> ParseSSEData(string sseText)
+    {
+        return sseText.Split('\n')
+            .Where(line => line.StartsWith("data: "))
+            .Select(line => line.Substring(6).Trim())
+            .ToList();
+    }
+}
+
+/// <summary>
+/// Simple mock HttpMessageHandler that returns a canned response.
+/// </summary>
+public class MockHttpMessageHandler : HttpMessageHandler
+{
+    private readonly HttpResponseMessage _response;
+
+    public MockHttpMessageHandler(HttpResponseMessage response)
+    {
+        _response = response;
+    }
+
+    protected override Task<HttpResponseMessage> SendAsync(
+        HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        return Task.FromResult(_response);
+    }
+}