Parse structured outputs when retrieving background Responses

[jbeckwith-oai]

Carried over from #126 and originally requested by @agrobbin.

Structured-output parsing works when creating a Response synchronously and when resuming a response stream with the model supplied again. It is still not available for the ordinary non-streaming background retrieval path: client.responses.retrieve(response_id) does not accept a structured-output model, so retrieved output cannot be coerced back into that model through content.parsed.

Desired behavior

Allow callers to supply the original structured-output model when retrieving a stored/background Response, for example:

response = client.responses.retrieve(
  response_id,
  text: CalendarEvent
)

The exact API can differ, but it should:

• parse matching output content into the supplied model;
• support tool models consistently where applicable;
• leave retrieval without a model backward-compatible;
• document the background create/poll/retrieve flow; and
• include tests for completed and not-yet-completed background responses.

Related partial support: structured-output parsing is already available when resuming streams. Source: #126.