Stream Temporal engine turn progress over /invoke instead of going silent until completion

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Summary

Reported live: a chat turn routed through the Temporal engine showed no text streaming and no SSE progress events — nothing until the whole turn completed.

TemporalEngine.stream() was the cause, and it was a documented, deliberate simplification, not a regression: it just awaited invoke() (a full accept/poll loop to completion) and yielded exactly one final chunk. Its own comment recorded the tradeoff — "a streaming client on this engine therefore sees the reply rather than the running commentary."

The engine's own Go gateway already solves this problem for its native /v1/chat/completions SSE endpoint: streamTurn polls workflows.TurnProgressQuery (a Temporal query exposing the workflow's own in-flight narration buffer) every 700ms and streams new lines as status events. agent-orchestrator never talks to that endpoint, though — Open WebUI's openaiBaseApiUrl points at agent-orchestrator, which always routes through the generic accept/poll /invoke contract via TemporalEngine.ts, and that contract had no equivalent capability.

Fix

Surfaces the same TurnProgressQuery data over /invoke too, rather than building a second mechanism:

• GET /invoke/:id's existing "pending" response now includes a progress field — the same narration lines the native endpoint already streams from. Best-effort: a query failure or an inactive turn just means an empty list, never an error, since the pending status itself is still accurate.
• TemporalEngine.poll() tracks how many lines it has already relayed and calls the turn's progressListener for each new one as it arrives — the exact callback agent-orchestrator's own SSE writer (handleChatCompletionsStreaming) already listens to for the in-process LangGraph engine, so no changes were needed on the consuming side at all.

Test plan

• New unit tests: relays only new lines across repeated polls (no duplicates), and confirms nothing is called when the caller has no live channel (no progressListener set)
• go build ./..., go vet ./..., gofmt -l ., go test ./... clean in engines/temporal
• npx tsc --noEmit, full agent-orchestrator vitest suite clean (2 pre-existing, unrelated failures in nats-agent-channel*.test.ts — a different subsystem, confirmed unaffected by this change)
• e2e (bridged-agent-workflow, happy-path, caller-tools) still passes against minikube after rebuilding both the gateway and orchestrator images