feat: add retry middleware with per-attempt governance orchestration

src/polygateway/middleware/ratelimit.py

@@ -0,0 +1,46 @@
+"""限流准入封装: 后端故障必须报错而非放行(降级方向铁律)。
+
+非降级设计说明: 缓存/遥测挂 → 静默;限流挂 → 若放行,多 worker 会同时
+失去闸门直接击穿网关,故一律 GovernanceBackendError 上抛。
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from polygateway.errors import GovernanceBackendError
+
+if TYPE_CHECKING:
+    from polygateway.ports import Permit, RateLimiter
+    from polygateway.types import SourceConfig, SourceStats
+
+
+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:
+        self._limiter = limiter
+
+    async def try_acquire(self, source: SourceConfig) -> Permit | None:
+        try:
+            return await self._limiter.try_acquire(source.name, source.est_tokens)
+        except GovernanceBackendError:
+            raise
+        except Exception as exc:
+            raise GovernanceBackendError(f"限流后端故障(try_acquire): {exc}") from exc
+
+    async def stats(self, source: SourceConfig) -> SourceStats:
+        try:
+            return await self._limiter.source_stats(source.name)
+        except GovernanceBackendError:
+            raise
+        except Exception as exc:
+            raise GovernanceBackendError(f"限流后端故障(source_stats): {exc}") from exc
+
+    async def mark_progress(self) -> None:
+        try:
+            await self._limiter.mark_progress()
+        except GovernanceBackendError:
+            raise
+        except Exception as exc:
+            raise GovernanceBackendError(f"限流后端故障(mark_progress): {exc}") from exc