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/backends/redis/breaker.py

@@ -367,7 +367,7 @@ class RedisGate:
                 keys=[self._key(source_name)], args=[owner, self._probe_ttl_ms]
             )
         except RedisError as exc:
-            raise GovernanceBackendError(f"熔断后端 try_enter 失败: {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端 try_enter 失败: {exc}", scope=self._scope) from exc
         return self._decision(source_name, result)
 
     async def record_success(
@@ -385,7 +385,7 @@ class RedisGate:
         try:
             result = await self._success_lua(keys=[self._key(entry.source_name)], args=args)
         except RedisError as exc:
-            raise GovernanceBackendError(f"熔断后端 record_success 失败: {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端 record_success 失败: {exc}", scope=self._scope) from exc
         return self._update(result)
 
     async def record_failure(
@@ -407,7 +407,7 @@ class RedisGate:
         try:
             result = await self._failure_lua(keys=[self._key(entry.source_name)], args=args)
         except RedisError as exc:
-            raise GovernanceBackendError(f"熔断后端 record_failure 失败: {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端 record_failure 失败: {exc}", scope=self._scope) from exc
         return self._update(result)
 
     async def release_probe(self, entry: GateDecision) -> GateUpdate:
@@ -419,7 +419,7 @@ class RedisGate:
                 keys=[self._key(entry.source_name)], args=[entry.epoch, entry.probe_owner]
             )
         except RedisError as exc:
-            raise GovernanceBackendError(f"熔断后端 release_probe 失败: {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端 release_probe 失败: {exc}", scope=self._scope) from exc
         return self._update(result)
 
     async def retry_after_s(self, sources: tuple[str, ...]) -> float:
@@ -429,7 +429,7 @@ class RedisGate:
         try:
             result = await self._retry_after_lua(keys=[self._key(s) for s in sources])
         except RedisError as exc:
-            raise GovernanceBackendError(f"熔断后端 retry_after_s 失败: {exc}") from exc
+            raise GovernanceBackendError(f"熔断后端 retry_after_s 失败: {exc}", scope=self._scope) from exc
         return int(result) / 1000.0
 
     async def aclose(self) -> None: