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/unit/test_backpressure.py

@@ -11,7 +11,13 @@ import pytest
 
 from polygateway.backends.memory.breaker import InMemoryGate
 from polygateway.backends.memory.limiter import InMemoryLimiter
-from polygateway.errors import AllSourcesExhausted, GovernanceBackendError, TransientError
+from polygateway.errors import (
+    AllSourcesExhausted,
+    GatewayUnavailableError,
+    GovernanceBackendError,
+    SourceNotConfiguredError,
+    TransientError,
+)
 from polygateway.middleware.retry import RetryMW, backoff_delay
 from polygateway.sources import RoundRobinSelector, SourceCooldownMemo
 from polygateway.types import (
@@ -173,17 +179,17 @@ class TestStallQuadrants:
 
 class _GateSuccessBroken(InMemoryGate):
     async def record_success(self, entry):
-        raise GovernanceBackendError("redis 抖动")
+        raise GovernanceBackendError("redis 抖动", scope="llm")
 
 
 class _GateFailureBroken(InMemoryGate):
     async def record_failure(self, entry, reason, force_open):
-        raise GovernanceBackendError("redis 抖动")
+        raise GovernanceBackendError("redis 抖动", scope="llm")
 
 
 class _LimiterProgressBroken(InMemoryLimiter):
     async def mark_progress(self):
-        raise GovernanceBackendError("redis 抖动")
+        raise GovernanceBackendError("redis 抖动", scope="llm")
 
 
 class TestAccountingDegradation:
@@ -252,6 +258,71 @@ class TestQuotaGateProgressAge:
             async def progress_age_s(self):
                 raise OSError("down")
 
-        assert await QuotaGate(_L()).progress_age_s() == 12.5
+        assert await QuotaGate(_L(), scope="llm").progress_age_s() == 12.5
         with pytest.raises(GovernanceBackendError):
-            await QuotaGate(_Broken()).progress_age_s()
+            await QuotaGate(_Broken(), scope="llm").progress_age_s()
+
+
+class TestUnknownSourceIsAssemblyDefect:
+    """未知源 = 限流后端的源名单与治理循环对不上,是装配缺陷不是后端故障。
+
+    两个后端行为必须一致(Redis 版对应用例在 `test_redis_key_layout.py::
+    TestConversions::test_unknown_source_rejected`);内存版此前无覆盖,
+    该分支从未被测过(issue #7 §3.4)。
+    """
+
+    def test_memory_limiter_rejects_unknown_source(self):
+        limiter = InMemoryLimiter(
+            scope="llm", sources={"s1": make_source("s1")}, global_limits=_NO_GLOBAL
+        )
+        with pytest.raises(SourceNotConfiguredError) as ei:
+            limiter._cfg("nope")
+        # 关键: 若归入 scope 级家族,配置写错的任务会永远延期重投、永不进死信
+        assert not isinstance(ei.value, GatewayUnavailableError)
+
+
+class TestGateFailuresReachCallersAsScopeLevel:
+    """三条闸门泄漏路径必须以 scope 级不可用的形态到达调用方(issue #7)。
+
+    记账路径由 `_record_quietly` 降级为 warning,但闸门路径没有那层包裹,会一路
+    抛给调用方。只写 `except GatewayUnavailableError` 的调用方此前接不住,后果
+    是 Redis 抖一下就让积压任务烧掉业务失败预算进死信——而那是运维重启即可恢复
+    的故障。三条路径逐一钉住,防止将来任何一条被漏掉。
+    """
+
+    async def test_try_acquire_failure_is_scope_level(self):
+        from polygateway.middleware.ratelimit import QuotaGate
+
+        class _Broken:
+            async def try_acquire(self, name, est):
+                raise OSError("down")
+
+        with pytest.raises(GatewayUnavailableError) as ei:
+            await QuotaGate(_Broken(), scope="LLM").try_acquire(make_source("s1"))
+        assert ei.value.scope == "llm"
+        assert ei.value.reason == "governance_backend_down"
+        assert ei.value.retry_after_s > 0  # 0 会让积压任务零延迟冲击已挂的后端
+
+    async def test_try_enter_failure_is_scope_level(self):
+        from polygateway.middleware.breaker import BreakerGate
+
+        class _Broken:
+            async def try_enter(self, name, owner):
+                raise OSError("down")
+
+        with pytest.raises(GatewayUnavailableError) as ei:
+            await BreakerGate(_Broken(), scope="LLM").try_enter(make_source("s1"), "owner")
+        assert ei.value.scope == "llm"
+        assert ei.value.reason == "governance_backend_down"
+
+    async def test_progress_age_failure_is_scope_level(self):
+        from polygateway.middleware.ratelimit import QuotaGate
+
+        class _Broken:
+            async def progress_age_s(self):
+                raise OSError("down")
+
+        with pytest.raises(GatewayUnavailableError) as ei:
+            await QuotaGate(_Broken(), scope="LLM").progress_age_s()
+        assert ei.value.scope == "llm"
+        assert ei.value.reason == "governance_backend_down"