fix: Gemini system prompt (#381)

I started to get 400 Bad Request from Gemini API, not sure if it was due
to changing a model or what. Changing `system_instruction.parts` into a
list like in the
[docs](https://ai.google.dev/gemini-api/docs/text-generation#system-instructions)
fixed it.

Tested at least with `gemini-1.5-pro` and `gemini-2.5-pro` and
`gemini-2.0-flash`, works fine.

Author: valtzu

src/platform/src/Bridge/Google/Contract/MessageBagNormalizer.php

@@ -44,7 +44,7 @@ protected function supportsModel(Model $model): bool
      *          role: 'model'|'user',
      *          parts: array<int, mixed>
      *      }>,
-     *      system_instruction?: array{parts: array{text: string}}
+     *      system_instruction?: array{parts: array{text: string}[]}
      *  }
      */
     public function normalize(mixed $data, ?string $format = null, array $context = []): array
@@ -53,7 +53,7 @@ public function normalize(mixed $data, ?string $format = null, array $context =
 
         if (null !== $systemMessage = $data->getSystemMessage()) {
             $array['system_instruction'] = [
-                'parts' => ['text' => $systemMessage->content],
+                'parts' => [['text' => $systemMessage->content]],
             ];
         }
 

src/platform/tests/Bridge/Google/Contract/MessageBagNormalizerTest.php

@@ -149,7 +149,7 @@ public static function provideMessageBagData(): iterable
                     ['role' => 'user', 'parts' => [['text' => 'Hello there']]],
                 ],
                 'system_instruction' => [
-                    'parts' => ['text' => 'You are a cat. Your name is Neko.'],
+                    'parts' => [['text' => 'You are a cat. Your name is Neko.']],
                 ],
             ],
         ];