[bug] OCI OpenAI models reject top-level ListProperty structured outputs #208

Description

In the current Wayflow version, a PromptExecutionStep cannot use a single ListProperty as its outermost output_descriptors entry when the model is an OpenAI model served through OCI's native chat API.

Wayflow sends a root JSON array schema for that output. OCI rejects the request with HTTP 400 for openai.gpt-5.4. An object schema containing the same list is accepted, so callers currently have to add an outer ObjectProperty to get the request through.

Reproduction

Configure an OCIGenAIModel with model_id="openai.gpt-5.4" and OCI's native API. Use it with a PromptExecutionStep configured for structured generation and one list output:

output_descriptors=[
    ListProperty(name="results", item_type=StringProperty())
]

Run the step. OCI rejects the root array schema with HTTP 400. Wrap the list in an ObjectProperty and the request succeeds.

Expected behavior

A caller should be able to declare a top-level ListProperty and receive a list without adding an outer object solely to satisfy OCI. This should work for both regular and streaming generation.

The workaround should apply to OpenAI models on OCI's native chat API without changing the schemas sent for other OCI model providers or for object outputs.

Environment

• Wayflow: current main before the proposed fix
• Model: openai.gpt-5.4
• API: OCI native chat API