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_redis_key_layout.py

@@ -68,10 +68,13 @@ class TestConversions:
             _limiter(lease_ttl_s=0)
 
     def test_unknown_source_rejected(self):
-        from polygateway.errors import GovernanceBackendError
+        """未知源是装配缺陷,不是后端故障(issue #7 §3.4)。"""
+        from polygateway.errors import GatewayUnavailableError, SourceNotConfiguredError
 
-        with pytest.raises(GovernanceBackendError):
+        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 TestLuaFidelity: