feat: add the {SCOPE}__CIRCUIT_OPEN admission policy key

Limiter rejections have always chosen between waiting and failing fast;
breaker rejections had no such choice. The new key is the missing cell
of that matrix, shaped exactly like QUOTA_FULL so there is nothing new
to learn. It defaults to fail_fast: flipping the default would move
every existing deployment's worst-case wall clock from milliseconds to
the stall window, which is the wrong direction to impose on anyone.
Single-source scopes are the ones that want wait, and they now have a
way to say so.

The two keys stay separate despite sharing a domain, because a full
quota is "queue for your share" (your turn always comes) while an open
circuit is "wait for the source to recover" (it might not).

Policy validation collapses into SourceAdmission, the only consumer.
The three client constructors used to each carry their own copy of the
quota_full check; adding a second key there would have made eight
copies of the same two lines. Rejection timing and message are
unchanged -- admission is built inside those constructors.

This commit only wires the key through; the control flow that reads it
lands next.

src/polygateway/client.py

@@ -134,6 +134,7 @@ class GatewayClient:
         retry: RetryPolicy,
         backpressure: BackpressurePolicy,
         quota_full: str = "wait",
+        circuit_open: str = "fail_fast",
         telemetry: TelemetryRecorder | None = None,
         pricing: PricingTable | None = None,
         text_cap: int | None = None,
@@ -162,6 +163,7 @@ class GatewayClient:
             retry=retry,
             backpressure=backpressure,
             quota_full=quota_full,
+            circuit_open=circuit_open,
             cooldown_memo=SourceCooldownMemo(now=now),
             # AIMD ceiling 尊重源级静态并发上限(独立核验 I1: 不得静默钳制大于 64 的配置)
             pacer=AdaptivePacer(
@@ -313,6 +315,7 @@ class GatewayClient:
             retry=settings.retry,
             backpressure=settings.backpressure,
             quota_full=settings.quota_full,
+            circuit_open=settings.circuit_open,
             telemetry=telemetry if telemetry is not None else _build_telemetry(settings),
             pricing=PricingTable.from_file(settings.pricing_path)
             if settings.pricing_path is not None