Add simple benchmark test for GPT-5 reasoning efforts

Author: kzu

src/AI.Tests/OpenAITests.cs

@@ -119,6 +119,54 @@ public async Task GPT5_ThinksFast()
             Assert.Equal("minimal", search["effort"]?.GetValue<string>());
         });
     }
+
+    [SecretsTheory("OPENAI_API_KEY")]
+    [InlineData(ReasoningEffort.Minimal)]
+    [InlineData(ReasoningEffort.Low)]
+    [InlineData(ReasoningEffort.Medium)]
+    [InlineData(ReasoningEffort.High)]
+    public async Task GPT5_ThinkingTime(ReasoningEffort effort)
+    {
+        var messages = new Chat()
+        {
+            { "system", "You are an intelligent AI assistant that's an expert on financial matters." },
+            { "user", "If you have a debt of 100k and accumulate a compounding 5% debt on top of it every year, how long before you are a negative millonaire? (round up to full integer value)" },
+        };
+
+        var requests = new List<JsonNode>();
+
+        var chat = new OpenAIChatClient(Configuration["OPENAI_API_KEY"]!, "gpt-5-nano",
+            OpenAIClientOptions.Observable(requests.Add).WriteTo(output));
+
+        var options = new ChatOptions
+        {
+            ModelId = "gpt-5",
+            ReasoningEffort = effort
+        };
+
+        var watch = System.Diagnostics.Stopwatch.StartNew();
+        var response = await chat.GetResponseAsync(messages, options);
+        watch.Stop();
+
+        var text = response.Text;
+
+        Assert.Contains("48 years", text);
+        // NOTE: the chat client was requested as grok-3 but the chat options wanted a 
+        // different model and the grok client honors that choice.
+        Assert.StartsWith("gpt-5", response.ModelId);
+        Assert.DoesNotContain("nano", response.ModelId);
+
+        // Reasoning should have been set to medium
+        Assert.All(requests, x =>
+        {
+            var search = Assert.IsType<JsonObject>(x["reasoning"]);
+            Assert.Equal(effort.ToString().ToLowerInvariant(), search["effort"]?.GetValue<string>());
+        });
+
+        output.WriteLine($"Effort: {effort}, Time: {watch.ElapsedMilliseconds}ms, Tokens: {response.Usage?.TotalTokenCount}");
+    }
+
+
     [SecretsFact("OPENAI_API_KEY")]
     public async Task WebSearchCountryHighContext()
     {