Default `matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_size` to `null`

Ref: 91986a129c/CHANGELOG.md (2025-05-11-version-172)

roles/custom/matrix-bot-baibot/defaults/main.yml

@@ -391,7 +391,7 @@ matrix_bot_baibot_config_agents_static_definitions_openai_config_text_to_speech_
 matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_enabled: true
 matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_model_id: gpt-image-1
 matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_style: null
-matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_size: 1024x1024
+matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_size: null
 matrix_bot_baibot_config_agents_static_definitions_openai_config_image_generation_quality: null
 
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