feat(parser): add support for output_text in response parsing

[alessiopelliccione]

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Changes being requested

While consuming streaming responses (e.g. client.responses.stream(...)), the final snapshot sometimes arrived without an output_text field, so stream.finalResponse() returned an object where output_text was undefined.
This meant downstream code had to manually concatenate deltas, even though non-streaming responses already provide output_text.

The fix ensures both maybeParseResponse branches rebuild the text when the payload lacks it (or sets it to null), using a small shared helper around the existing addOutputText logic.

Additional context & links

Fixes #1662