test(config): lock model presets alias serialization

tests/config/test_model_presets.py

@@ -219,6 +219,23 @@ def test_model_presets_accepts_camel_case_root_key() -> None:
     assert config.model_presets["fast"].provider == "openai"
 
 
+def test_model_presets_serializes_with_camel_case_root_key() -> None:
+    config = Config.model_validate({
+        "model_presets": {
+            "fast": {
+                "model": "openai/gpt-4.1",
+                "provider": "openai",
+            }
+        },
+    })
+
+    dumped = config.model_dump(mode="json", by_alias=True)
+
+    assert "modelPresets" in dumped
+    assert "model_presets" not in dumped
+    assert dumped["modelPresets"]["fast"]["model"] == "openai/gpt-4.1"
+
+
 def test_resolve_preset_can_target_named_preset_without_activating() -> None:
     config = Config.model_validate({
         "model_presets": {