Remove unsupported response_mime_type parameter from Gemini API call

- The old google.generativeai package doesn't support image/png in response_mime_type
- Images are returned naturally in response parts as inline_data
- This fixes the 400 error: allowed mimetypes are text/plain, application/json, etc.

src/core.py

@@ -163,15 +163,11 @@ def _call_gemini_edit(
         {"mime_type": "image/png", "data": mask_bytes},
     ]
 
-    # Request image response explicitly
-    generation_config = {
-        "response_mime_type": "image/png",
-    }
-    
+    # Note: response_mime_type doesn't support image/png in the old google.generativeai package
+    # Images are returned in response parts as inline_data
     try:
         response = model.generate_content(
             content,
-            generation_config=generation_config,
             stream=False
         )
     except Exception as gen_err: