fix(image_gen): use gpt-5.5 for Codex image host

plugins/image_gen/openai-codex/__init__.py

@@ -73,7 +73,7 @@ _SIZES = {
 # Codex Responses surface used for the request. The chat model itself is only
 # the host that calls the ``image_generation`` tool; the actual image work is
 # done by ``API_MODEL``.
-_CODEX_CHAT_MODEL = "gpt-5.4"
+_CODEX_CHAT_MODEL = "gpt-5.5"
 _CODEX_BASE_URL = "https://chatgpt.com/backend-api/codex"
 _CODEX_INSTRUCTIONS = (
     "You are an assistant that must fulfill image generation requests by "

tests/plugins/image_gen/test_openai_codex_provider.py

@@ -142,7 +142,7 @@ class TestGenerate:
         result = provider.generate("a cat", aspect_ratio="portrait")
         assert result["success"] is True
 
-        assert captured["model"] == "gpt-5.4"
+        assert captured["model"] == "gpt-5.5"
         assert captured["store"] is False
         assert captured["input"][0]["type"] == "message"
         assert captured["input"][0]["role"] == "user"