Structured output streaming: no way to see the object while it generates

Problem

With output_type=<pydantic model>, ai.stream() emits text deltas but the parsed object only exists after the stream ends (stream.output). Consumers rendering progressive results — live forms, tables, status panels — cannot show the object taking shape. The TypeScript AI SDK's streamObject() supports this and it's a common expectation coming from that ecosystem (pydantic-ai streams validated partials as well).

Suggested behavior

Emit a PartialOutput event after each text delta that changes the parse of the accumulated JSON, carrying the best-effort partial as a plain dict, plus a Stream.partial_output property for the latest snapshot.

Design notes:

• partials are deliberately unvalidated dicts — validating incomplete JSON is meaningless; stream.output stays the validated contract
• parsing uses json_repair with stream_stable=True so truncated values remain verbatim strings instead of being creatively repaired mid-stream
• agent runs forward model stream events unchanged, so this surfaces through Agent.run for free

Happy to take this one.