feat: Add Nebius AI as a CustomProvider

Add Nebius AI Studio (https://api.studio.nebius.ai/v1/) as an
OpenAI-compatible custom provider with test infrastructure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: HavenDV

src/libs/tryAGI.OpenAI/CustomProviders.cs

@@ -91,6 +91,11 @@ public static class CustomProviders
     /// </summary>
     public const string CerebrasBaseUrl = "https://api.cerebras.ai/v1";
 
+    /// <summary>
+    /// https://docs.nebius.com/studio/inference/api
+    /// </summary>
+    public const string NebiusBaseUrl = "https://api.studio.nebius.ai/v1/";
+
     /// <summary>
     /// Creates an API to use for GitHub Models: https://github.com/marketplace/models
     /// </summary>
@@ -252,4 +257,13 @@ public static OpenAiClient Cerebras(string apiKey)
     {
         return new OpenAiClient(apiKey, baseUri: new Uri(CerebrasBaseUrl));
     }
+
+    /// <summary>
+    /// Create an API to use for Nebius AI Studio.
+    /// </summary>
+    /// <returns></returns>
+    public static OpenAiClient Nebius(string apiKey)
+    {
+        return new OpenAiClient(apiKey, baseUri: new Uri(NebiusBaseUrl));
+    }
 }

src/tests/OpenAI.IntegrationTests/CustomProvider.cs

@@ -21,4 +21,5 @@ public enum CustomProvider
     Hyperbolic,
     Cerebras,
     Cohere,
+    Nebius,
 }

src/tests/OpenAI.IntegrationTests/Tests.Helpers.cs

@@ -164,6 +164,13 @@ internal static (OpenAiClient Api, string Model) GetAuthorizedChatApi(CustomProv
                 throw new AssertInconclusiveException("COHERE_API_KEY environment variable is not found.")),
                 model ?? "command-r-08-2024");
         }
+        if (customProvider == CustomProvider.Nebius)
+        {
+            return (CustomProviders.Nebius(apiKey:
+                Environment.GetEnvironmentVariable("NEBIUS_API_KEY") ??
+                throw new AssertInconclusiveException("NEBIUS_API_KEY environment variable is not found.")),
+                model ?? "meta-llama/Meta-Llama-3.1-8B-Instruct");
+        }
 
         var apiKey =
             Environment.GetEnvironmentVariable("OPENAI_API_KEY") ??