Streaming: chat_stream + SSE endpoint

[sagi5060]

Problem

chat() blocks until the full turn completes. Middle's UX targets (first acknowledgement p95 < 5s) need incremental output; today a customer stares at nothing for the whole turn.

Proposed shape

• app.chat_stream(name, session_id, message) → async iterator of text deltas (wraps SDK Runner.run_streamed), same session semantics as chat().
• POST /agents/{name}/chat grows ?stream=true (or an SSE sibling endpoint) emitting text/event-stream deltas + a final done event with the full output.

Done when

• Deltas arrive before the turn completes (test with a mocked model or a live key).
• Session history is identical whether the turn was streamed or not.
• Non-streaming path unchanged.