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.

tests/integration/test_redis_cross_connection.py

@@ -16,7 +16,11 @@ import pytest
 from polygateway.backends.redis.breaker import RedisGate
 from polygateway.backends.redis.limiter import RedisLimiter
 from polygateway.client import GatewayClient
-from polygateway.errors import AllSourcesExhausted, GovernanceBackendError
+from polygateway.errors import (
+    AllSourcesExhausted,
+    GatewayUnavailableError,
+    GovernanceBackendError,
+)
 from polygateway.sources import RoundRobinSelector
 from polygateway.types import (
     BackpressurePolicy,
@@ -225,7 +229,11 @@ async def test_cancel_in_flight_releases_lease(clients):
 
 
 async def test_redis_down_admission_fails_closed():
-    """Redis 不可达 → 准入侧抛 GovernanceBackendError,绝不放行(库铁律)。"""
+    """Redis 不可达 → 准入侧报错绝不放行(库铁律),且以 scope 级形态到达调用方。
+
+    issue #7: 调用方只写 `except GatewayUnavailableError` 就该覆盖后端故障——
+    真实 Redis 掉线是这条链路唯一的端到端证据,故断言收紧到 scope 级语义。
+    """
     import redis.asyncio as aioredis
 
     dead = aioredis.from_url(
@@ -240,9 +248,12 @@ async def test_redis_down_admission_fails_closed():
             lease_ttl_s=30.0,
         )
         gate = RedisGate(config=_CFG, redis=dead, scope="t-dead")
-        with pytest.raises(GovernanceBackendError):
-            await limiter.try_acquire("s1", 0)
-        with pytest.raises(GovernanceBackendError):
-            await gate.try_enter("s1", "w")
+        for call in (limiter.try_acquire("s1", 0), gate.try_enter("s1", "w")):
+            with pytest.raises(GatewayUnavailableError) as ei:
+                await call
+            assert isinstance(ei.value, GovernanceBackendError)
+            assert ei.value.reason == "governance_backend_down"
+            assert ei.value.scope == "t-dead"
+            assert ei.value.retry_after_s > 0
     finally:
         await dead.aclose()