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/embedding.py

@@ -120,8 +120,8 @@ class EmbeddingClient:
         # 否则遥测会记录一个从未发出的采样参数(issue #4 决策 G)
         self._sources = strip_unsupported_extra_body(list(sources), path="embedding")
         self._selector = selector
-        self._quota = QuotaGate(limiter)
-        self._breaker = BreakerGate(breaker)
+        self._quota = QuotaGate(limiter, scope=self._scope)
+        self._breaker = BreakerGate(breaker, scope=self._scope)
         self._transport = transport
         self._retry = retry
         self._bp = backpressure