feat: map Worldtree b1 eager turn-launch statuses (409/503) in stream_turn
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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