Worldtree v1.0.0b1 (#331) decoupled turn execution from the SSE connection,
so turn-launch failures now arrive EAGERLY as an HTTP status before any
stream: 409 agent_not_available (pre-b1 was a 200 + in-stream error event)
and 503 (retryable turn-launch / infra failure). stream_turn previously
funneled both into a generic SseConnectFailed.
Map them to typed SseConnectFailed subclasses — AgentNotAvailable (409) and
TurnLaunchUnavailable (503, retryable=True) — carrying the parsed
error_code/message from the {detail:{error_code,message}} envelope.
Subclassing keeps existing `except SseConnectFailed` handlers working with
zero changes (POST-003 preserved — no synthetic event yielded; raise mirrors
reconnect_turn's 400/410/412 pattern).
worldtree-dev confirmed 409/503 are real runtime statuses; the OpenAPI 2.1.0
gap (not enumerating them) is theirs to fix (doc-completeness, not a wire
break). The 503 error_code is being re-pinned upstream (today internal_error
-> likely not_ready); our handling keys on STATUS so it's robust to the final
code — tighten the 503 default once they confirm.
Body shape live-confirmed against demo b1's 404/401 responses. Suite 509 green.
Contract docs/contracts/issues/1.contract.md updated.
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