Fix /health endpoint NotSupportedException from source-generated JSON serializer (#39)

* Fix /health endpoint NotSupportedException from source-generated JSON serializer

Replace anonymous type in /health endpoint with a proper HealthResponse
record registered in SkillServerJsonContext. The anonymous type was not
recognized by the source-generated serializer, causing a runtime crash.

* Assert /health response body in integration test

Add JSON deserialization and field-level assertions for status and
timestamp to the health endpoint test, catching serializer mismatches
earlier.

Author: Aaronontheweb

src/SkillServer/Endpoints.cs

@@ -50,7 +50,9 @@ private static void MapSkillEndpoints(this WebApplication app)
 
     private static void MapHealthEndpoints(this WebApplication app)
     {
-        app.MapGet("/health", () => Results.Ok(new { status = "healthy", timestamp = DateTimeOffset.UtcNow }));
+        app.MapGet("/health", () => Results.Json(
+            new HealthResponse { Status = "healthy", Timestamp = DateTimeOffset.UtcNow },
+            SkillServerJsonContext.Default.HealthResponse));
     }
 
     private static async Task<IResult> ListSkills(

src/SkillServer/Models/ApiModels.cs

@@ -186,3 +186,15 @@ public sealed record ApiKeySummary
     [JsonPropertyName("expiresAt")]
     public DateTimeOffset? ExpiresAt { get; init; }
 }
+
+/// <summary>
+/// Response from the health check endpoint.
+/// </summary>
+public sealed record HealthResponse
+{
+    [JsonPropertyName("status")]
+    public required string Status { get; init; }
+
+    [JsonPropertyName("timestamp")]
+    public required DateTimeOffset Timestamp { get; init; }
+}

src/SkillServer/Models/SkillServerJsonContext.cs

@@ -26,6 +26,7 @@ namespace SkillServer.Models;
 [JsonSerializable(typeof(IReadOnlyList<ApiKeySummary>))]
 [JsonSerializable(typeof(IReadOnlyList<CheckUpdateRequestItem>))]
 [JsonSerializable(typeof(IReadOnlyList<CheckUpdateResponseItem>))]
+[JsonSerializable(typeof(HealthResponse))]
 [JsonSourceGenerationOptions(
     PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
     DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]

tests/SkillServer.Integration.Tests/SkillServerIntegrationTests.cs

@@ -30,6 +30,11 @@ public async Task Health_ReturnsOk()
         var response = await _fixture.HttpClient.GetAsync("/health", ct);
 
         Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+
+        var body = await response.Content.ReadFromJsonAsync<SkillServer.Models.HealthResponse>(ct);
+        Assert.NotNull(body);
+        Assert.Equal("healthy", body.Status);
+        Assert.True(body.Timestamp > DateTimeOffset.MinValue);
     }
 
     [Fact]