feat: add gateway client with env-driven assembly

Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.

src/polygateway/middleware/breaker.py

@@ -33,7 +33,9 @@ class BreakerGate:
         except Exception as exc:
             raise GovernanceBackendError(f"熔断后端故障(record_success): {exc}") from exc
 
-    async def record_failure(self, entry: GateDecision, reason: str, force_open: bool) -> GateUpdate:
+    async def record_failure(
+        self, entry: GateDecision, reason: str, force_open: bool
+    ) -> GateUpdate:
         try:
             return await self._gate.record_failure(entry, reason, force_open)
         except GovernanceBackendError: