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/middleware/breaker.py

@@ -14,8 +14,10 @@ if TYPE_CHECKING:
 class BreakerGate:
     """RetryMW 面向熔断后端的唯一入口;包装一切后端异常。"""
 
-    def __init__(self, gate: ProviderGate) -> None:
+    def __init__(self, gate: ProviderGate, *, scope: str) -> None:
         self._gate = gate
+        # 后端故障即 scope 级不可用,异常须携 scope 供调用方定位(issue #7 §3.3)
+        self._scope = scope
 
     async def try_enter(self, source: SourceConfig, owner: str) -> GateDecision:
         try:
@@ -23,7 +25,7 @@ class BreakerGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"熔断后端故障(try_enter): {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端故障(try_enter): {exc}", scope=self._scope) from exc
 
     async def record_success(
         self, entry: GateDecision, *, count_attempt: bool = True
@@ -33,7 +35,7 @@ class BreakerGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"熔断后端故障(record_success): {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端故障(record_success): {exc}", scope=self._scope) from exc
 
     async def record_failure(
         self, entry: GateDecision, reason: str, force_open: bool
@@ -43,7 +45,7 @@ class BreakerGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"熔断后端故障(record_failure): {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端故障(record_failure): {exc}", scope=self._scope) from exc
 
     async def release_probe(self, entry: GateDecision) -> GateUpdate:
         try:
@@ -51,7 +53,7 @@ class BreakerGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"熔断后端故障(release_probe): {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端故障(release_probe): {exc}", scope=self._scope) from exc
 
     async def retry_after_s(self, sources: tuple[str, ...]) -> float:
         try:
@@ -59,4 +61,4 @@ class BreakerGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"熔断后端故障(retry_after_s): {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端故障(retry_after_s): {exc}", scope=self._scope) from exc

src/polygateway/middleware/ratelimit.py

@@ -18,8 +18,10 @@ if TYPE_CHECKING:
 class QuotaGate:
     """RetryMW 面向限流后端的唯一入口;包装一切后端异常。"""
 
-    def __init__(self, limiter: RateLimiter) -> None:
+    def __init__(self, limiter: RateLimiter, *, scope: str) -> None:
         self._limiter = limiter
+        # 后端故障即 scope 级不可用,异常须携 scope 供调用方定位(issue #7 §3.3)
+        self._scope = scope
 
     async def try_acquire(self, source: SourceConfig) -> Permit | None:
         try:
@@ -27,7 +29,7 @@ class QuotaGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"限流后端故障(try_acquire): {exc}") from exc
+            raise GovernanceBackendError(f"限流后端故障(try_acquire): {exc}", scope=self._scope) from exc
 
     async def stats(self, source: SourceConfig) -> SourceStats:
         try:
@@ -35,7 +37,7 @@ class QuotaGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"限流后端故障(source_stats): {exc}") from exc
+            raise GovernanceBackendError(f"限流后端故障(source_stats): {exc}", scope=self._scope) from exc
 
     async def mark_progress(self) -> None:
         try:
@@ -43,7 +45,7 @@ class QuotaGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"限流后端故障(mark_progress): {exc}") from exc
+            raise GovernanceBackendError(f"限流后端故障(mark_progress): {exc}", scope=self._scope) from exc
 
     async def progress_age_s(self) -> float:
         try:
@@ -51,4 +53,4 @@ class QuotaGate:
         except GovernanceBackendError:
             raise
         except Exception as exc:
-            raise GovernanceBackendError(f"限流后端故障(progress_age_s): {exc}") from exc
+            raise GovernanceBackendError(f"限流后端故障(progress_age_s): {exc}", scope=self._scope) from exc

src/polygateway/middleware/retry.py

@@ -183,8 +183,8 @@ class RetryMW:
         self._scope = scope
         self._sources = list(sources)
         self._selector = selector
-        self._quota = QuotaGate(limiter)
-        self._breaker = BreakerGate(gate)
+        self._quota = QuotaGate(limiter, scope=self._scope)
+        self._breaker = BreakerGate(gate, scope=self._scope)
         self._transport = transport
         self._retry = retry
         self._bp = backpressure