Fix chat SSE timeout on long-running Temporal-engine turns

[imaustink]

Summary

Root-causes the chat-UI timeout ("Timeout or cancellation waiting for update: stream terminated by RST_STREAM with error code: CANCEL") seen on a long-running agent turn, even though the underlying agentrun-* Kubernetes Job kept running and completed successfully server-side.

TemporalEngine.stream() (apps/agent-orchestrator/src/engine/temporal-engine.ts) did await this.invoke(input) before returning its iterable. server.ts's streaming handler does:

const source = await this.graph.stream(graphInput, { streamMode: "updates" });
for await (const item of withHeartbeat(source, HEARTBEAT_MS)) { ... }

withHeartbeat exists specifically to emit an SSE keep-alive comment whenever the source stalls for longer than HEARTBEAT_MS (15s) — but it can't race against a source it doesn't have yet. Because stream()'s own Promise didn't settle until the entire turn (up to the 30-minute poll timeout) had already finished, the heartbeat wrapper never engaged for the whole duration of a Temporal-engine turn. The only bytes reaching the client during that window were opportunistic progress-narration writes (added in #208) — not a guaranteed cadence — so any lull (job scheduling/image pull, a long tool call, or simply the seconds before the first narration line) longer than an upstream idle-connection timeout got the stream cancelled.

Fix

Move the await this.invoke(input) inside the returned async generator's body instead of before the return. Async generator bodies don't run until the first .next() call, so stream() now resolves immediately, letting withHeartbeat start racing the real turn duration against its keep-alive timer from the start — exactly like it already does for the LangGraph engine.

No behavior change for a non-streaming caller (invoke() is untouched) or for what gets rendered — still a single terminal update once the turn settles.

Test plan

• Added a regression test (temporal-engine.test.ts) asserting stream() resolves promptly even while the underlying turn is still "hanging", and that iterating the returned source still drains through to the terminal result once the turn completes.
• npx vitest run src/engine/temporal-engine.test.ts — 12/12 passing.
• Full apps/agent-orchestrator suite run for regressions — the 2 failing files (nats-agent-channel*.test.ts) and the pre-existing tsc --noEmit errors in nats-agent-channel.ts reproduce identically on main without this change (unrelated, stale workspace-package typing issue).