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.

docs/contracts/issues/1.contract.md

@@ -106,7 +106,7 @@ POST: [POST-002 return_value] AsyncIterator yields ≥1 event ending in exactly
 POST: [POST-003 state_change] every yielded Event has a populated sse_id with both fields >= 1 -- assert all(e.sse_id.turn_id >= 1 and e.sse_id.seq >= 1 for e in events)
 ERROR_ROUTING:
   httpx.HTTPStatusError:
-    local_handling: re-raise as SseConnectFailed(status=resp.status_code, body=resp.read()[:1024]) — server returned non-2xx before stream started (e.g., 404 session_not_found)
+    local_handling: re-raise as SseConnectFailed(status=resp.status_code, body=resp.read()[:1024]) — server returned non-2xx before stream started (e.g., 404 session_not_found). EXCEPT the two eager turn-launch failures (Worldtree v1.0.0b1 #331), checked BEFORE raise_for_status and raised as typed SseConnectFailed SUBCLASSES carrying error_code: 409 -> AgentNotAvailable (agent unavailable; pre-b1 this was a 200 + in-stream `error` event), 503 -> TurnLaunchUnavailable (transient turn-launch failure; retryable=True). Subclassing keeps existing `except SseConnectFailed` handlers working with zero changes.
     flow_control: abort
     state_recovery: none (no events yielded yet)
   httpx.ReadError | httpx.RemoteProtocolError | httpx.ReadTimeout:
@@ -129,7 +129,8 @@ ERROR_ROUTING:
 STEPS:
   1. [setup, flexibility=prescriptive] Validate inputs per PRE-001..PRE-003
   2. [sequential, flexibility=prescriptive] Open SSE connection via httpx_sse.aconnect_sse with method="POST", url=f"/sessions/{session_id}/messages", json={"content": content}
-     ON httpx.HTTPStatusError before stream opens:
+     2a. [branch, flexibility=prescriptive] IF response.status_code in (409, 503) (b1 #331 eager turn-launch failures): read the body, parse (error_code, message) from the `{"detail": {...}}` envelope OR a flat `{error_code, message}` body (status-derived default code when absent), then RAISE AgentNotAvailable (409) / TurnLaunchUnavailable (503).
+     ON httpx.HTTPStatusError before stream opens (any other non-2xx):
        RAISE SseConnectFailed
   3. [loop, flexibility=prescriptive] FOR EACH sse_event in event_source.aiter_sse():
      0. [branch, flexibility=prescriptive] IF sse_event.data == "":
@@ -162,6 +163,9 @@ TESTS:
   error_terminal [error]: mock emits one `text` then `error` with `error_code: "llm_output_invalid"` → consumer yields Text then Error; iteration ends; Error.message and Error.error_code are populated
   cancelled_terminal [error]: mock emits `cancelled` with phase=cancelled → consumer yields Cancelled with turn_id; iteration ends
   session_not_found [error]: mock returns 404 before stream opens → consumer raises SseConnectFailed(status=404)
+  eager_409_agent_not_available [error]: mock returns 409 {detail:{error_code:"agent_not_available", message}} before stream → consumer raises AgentNotAvailable(status=409, error_code="agent_not_available", retryable absent); isinstance SseConnectFailed
+  eager_503_retryable [error]: mock returns 503 before stream → consumer raises TurnLaunchUnavailable(status=503, retryable=True); isinstance SseConnectFailed
+  eager_409_non_json_body [adversarial]: mock returns 409 with a non-JSON body → consumer raises AgentNotAvailable with the status-derived default error_code "agent_not_available"
   malformed_id_no_seq [adversarial]: mock event has `id: 42` (missing `:seq`) → consumer raises MalformedSseId; no event yielded
   malformed_id_alpha [adversarial]: mock event has `id: foo:bar` (non-integer parts) → consumer raises MalformedSseId
   turn_id_flip [adversarial]: mock emits text events with ids `42:1` then `99:2` → consumer raises TurnIdFlip; only the first event was yielded