fix: reparent governance backend failures under GatewayUnavailableError (issue #7)

A fail-closed limiter or breaker backend means the scope cannot emit a
single request, which is exactly scope-level unavailability. But the error
sat directly under PolyGatewayError, so a caller writing only
`except GatewayUnavailableError` dropped it into the catch-all branch:
Redis blips once and a backlog of tasks burns its business failure budget
into the dead letter queue, over a fault a restart would clear.

Three gate paths leak to callers rather than being absorbed by
_record_quietly (try_acquire, try_enter, progress_age_s); each is now
pinned by a test, since none of them had one before.

The two unknown-source sites move to SourceNotConfiguredError instead of
following along. They report a misconfigured source name, not an outage,
and letting them into the retryable family would be the mirror of the bug
being fixed here: the task would retry forever and never surface.

src/polygateway/errors.py

@@ -150,5 +150,29 @@ class SourceNotConfiguredError(PolyGatewayError):
     """
 
 
-class GovernanceBackendError(PolyGatewayError):
-    """限流/熔断状态后端自身故障: 必须报错而非放行(防击穿网关,降级方向铁律)。"""
+class GovernanceBackendError(GatewayUnavailableError):
+    """限流/熔断状态后端自身故障: 必须报错而非放行(防击穿网关,降级方向铁律)。
+
+    继承 `GatewayUnavailableError`(issue #7): fail-closed 意味着整个 scope 一个
+    请求都发不出去,语义上即 scope 级不可用。此前它是 `PolyGatewayError` 的直接
+    子类,只写 `except GatewayUnavailableError` 的调用方接不住,后果是"Redis 抖
+    一下 → 积压任务消耗业务失败预算 → 进死信",而那是运维重启即可恢复的故障。
+    """
+
+    def __init__(
+        self,
+        message: str,
+        *,
+        scope: str,
+        retry_after_s: float = GOVERNANCE_BACKEND_RETRY_AFTER_S,
+        source_name: str | None = None,
+    ) -> None:
+        super().__init__(
+            scope=scope,
+            reason="governance_backend_down",
+            retry_after_s=retry_after_s,
+            source_name=source_name,
+        )
+        # 父类会把 message 覆写为 "{scope} 网关暂时不可用: {reason}",而各构造点
+        # 携带的诊断串(如"限流后端 try_acquire 失败: ...")是排障主线索,必须保住
+        self.args = (message,)