Modify message structure to match OpenAI format.

Modifying the message structure to pass images along with text as the message content makes this work for generating image alt text using Ollama.

src/Plugin/AiProvider/OllamaProvider.php

@@ -218,19 +218,26 @@ class OllamaProvider extends AiProviderClientBase implements
       }
       /** @var \Drupal\ai\OperationType\Chat\ChatMessage $message */
       foreach ($input->getMessages() as $message) {
-
-        $images = [];
+        $content = [
+          [
+            'type' => 'text',
+            'text' => $message->getText(),
+          ],
+        ];
         if (count($message->getImages())) {
           foreach ($message->getImages() as $image) {
-            $images[] = $image->getAsBase64EncodedString('');
+            $content[] = [
+              'type' => 'image_url',
+              'image_url' => [
+                'url' => $image->getAsBase64EncodedString(),
+              ],
+            ];
           }
         }
-        $new_message = [
+        $chat_input[] = [
           'role' => $message->getRole(),
-          'content' => $message->getText(),
-          'images' => $images,
+          'content' => $content,
         ];
-        $chat_input[] = $new_message;
       }
     }
     $payload = [