feat(api, xai): integrate xAI Grok image models and extend API endpoints for image support

- Added new xAI Grok image models (`grok-imagine-image`, `grok-imagine-image-quality`) with high-fidelity and aspect ratio configurations.
- Extended `isSupportedImagesModel` logic to validate xAI models.
- Implemented API request builders for image generation/editing with customizable options (e.g., resolution, aspect ratio, response format).
- Enhanced `/v1/images` endpoints to handle xAI model capabilities, including response normalization and model-specific handlers.
- Updated unit tests to validate xAI model validation, request structure, and API integration.

internal/registry/models/models.json

@@ -46,8 +46,7 @@
         "levels": [
           "low",
           "medium",
-          "high",
-          "xhigh"
+          "high"
         ]
       }
     },
@@ -473,6 +472,30 @@
         "dynamic_allowed": true
       }
     },
+    {
+      "id": "gemini-2.5-flash-image",
+      "object": "model",
+      "created": 1763596800,
+      "owned_by": "google",
+      "type": "gemini",
+      "display_name": "Gemini 2.5 Flash Image",
+      "name": "models/gemini-2.5-flash-image",
+      "version": "001",
+      "description": "Our state-of-the-art image generation and editing model.",
+      "inputTokenLimit": 1048576,
+      "outputTokenLimit": 65536,
+      "supportedGenerationMethods": [
+        "generateContent",
+        "countTokens",
+        "createCachedContent",
+        "batchGenerateContent"
+      ],
+      "thinking": {
+        "max": 24576,
+        "zero_allowed": true,
+        "dynamic_allowed": true
+      }
+    },
     {
       "id": "gemini-2.5-flash-lite",
       "object": "model",
@@ -1990,12 +2013,12 @@
       }
     },
     {
-      "id": "gemini-3.1-pro-high",
+      "id": "gemini-pro-agent",
       "object": "model",
       "owned_by": "antigravity",
       "type": "antigravity",
       "display_name": "Gemini 3.1 Pro (High)",
-      "name": "gemini-3.1-pro-high",
+      "name": "gemini-pro-agent",
       "description": "Gemini 3.1 Pro (High)",
       "context_length": 1048576,
       "max_completion_tokens": 65535,