update MEAI image generation support and basic usage tracking for streaming

src/GenerativeAI.Microsoft/AdditionalPropertyKeys.cs

@@ -14,4 +14,14 @@ public static class AdditionalPropertiesKeys
     /// Key used to indicate the thinking budget in tokens.
     /// </summary>
     public const string ThinkingBudget = "ThinkingBudget";
+
+    /// <summary>
+    /// Key used to specify response modalities (e.g., text, image).
+    /// </summary>
+    public const string ResponseModalities = "ResponseModalities";
+
+    /// <summary>
+    /// Key used to specify the aspect ratio for image generation.
+    /// </summary>
+    public const string ImageConfigAspectRatio = "AspectRatio";
 }

src/GenerativeAI.Microsoft/Extensions/MicrosoftExtensions.cs

@@ -337,6 +337,17 @@ private static JsonNode ToJsonNodeResponse(this object? response)
             config.ThinkingConfig.IncludeThoughts = includeThoughts;
         }
 
+        if (options.AdditionalProperties.TryGetValue(AdditionalPropertiesKeys.ResponseModalities, out List<Modality>? modalities))
+        {
+            config.ResponseModalities = modalities;
+        }
+
+        if (options.AdditionalProperties.TryGetValue(AdditionalPropertiesKeys.ImageConfigAspectRatio, out string? aspectRatio))
+        {
+            config.ImageConfig ??= new ImageConfig();
+            config.ImageConfig.AspectRatio = aspectRatio;
+        }
+
         return config;
     }
 
@@ -394,9 +405,18 @@ public static ChatResponseUpdate ToChatResponseUpdate(this GenerateContentRespon
 
         if (response.Candidates != null)
         {
+            var contents = response.Candidates.Select(s => s.Content).SelectMany(s => s?.Parts ?? new List<Part>()).ToList().ToAiContents(options);
+
+            // Add usage information as UsageContent so it gets aggregated by the framework
+            var usage = ParseContentResponseUsage(response);
+            if (usage != null)
+            {
+                contents.Add(new UsageContent(usage));
+            }
+
             return new ChatResponseUpdate
             {
-                Contents = response.Candidates.Select(s => s.Content).SelectMany(s => s?.Parts ?? new List<Part>()).ToList().ToAiContents(options),
+                Contents = contents,
                 AdditionalProperties = null,
                 FinishReason = response?.Candidates?.FirstOrDefault()?.FinishReason == FinishReason.OTHER
                     ? ChatFinishReason.Stop

src/GenerativeAI/Types/ContentGeneration/Config/GenerationConfig.cs

@@ -209,6 +209,12 @@ public class GenerationConfig
     /// </summary>
     [JsonPropertyName("responseJsonSchema")]
     public JsonNode? ResponseJsonSchema { get; set; }
+
+    /// <summary>
+    /// Optional. Image generation configuration for models that support image output.
+    /// </summary>
+    [JsonPropertyName("imageConfig")]
+    public ImageConfig? ImageConfig { get; set; }
 }
 
 
@@ -321,4 +327,16 @@ public class ManualRoutingMode
     /// </summary>
     [JsonPropertyName("modelName")]
     public string? ModelName { get; set; }
+}
+
+/// <summary>
+/// Configuration for image generation in models that support image output (e.g., gemini-2.5-flash-image).
+/// </summary>
+public class ImageConfig
+{
+    /// <summary>
+    /// Optional. The aspect ratio for generated images. Common values: "16:9", "9:16", "1:1", "4:3", "3:4".
+    /// </summary>
+    [JsonPropertyName("aspectRatio")]
+    public string? AspectRatio { get; set; }
 }