Resolves #1525.

## Assistants
### New Features
* Introduces support for the new phase parameter in assistant responses.
Starting with GPT-5.4, the Responses API will mark an assistant message
as intermediate commentary or the final answer. The model uses the phase
parameter to distinguish working commentary from the completed answer in
previous responses. This matters most in multi-step flows with
preambles, tool-related updates, or multiple assistant messages in the
same turn. Missing or dropped phase can cause preambles to be treated as
final answers, which can degrade quality on multi-step tasks.