feat: suppress consecutive-channel opening on evidently healthy sources

Round 8 forensics caught the healthy source circuit-opened by five
random empty completions (~20% ambient failure rate makes a 5-streak
land every ~3000 attempts), blacking out the only good source for 60s.
When the window holds min_calls samples below the failure-rate
threshold, a streak is noise and no longer opens the gate; cold-start
and low-traffic semantics are unchanged and sudden death of a warm
source is still caught by the rate channel.

src/polygateway/backends/redis/breaker.py

@@ -179,6 +179,7 @@ end
 
 local threshold = tonumber(ARGV[5])
 local open_via_rate = false
+local window_healthy = false
 if force_open == 0 and is_probe == 0 then
     rotate_window(KEYS[1], now, tonumber(ARGV[10]))
     redis.call('HINCRBY', KEYS[1], 'a0', 1)
@@ -187,8 +188,13 @@ if force_open == 0 and is_probe == 0 then
         + tonumber(redis.call('HGET', KEYS[1], 'a1') or '0')
     local fails_w = tonumber(redis.call('HGET', KEYS[1], 'f0') or '0')
         + tonumber(redis.call('HGET', KEYS[1], 'f1') or '0')
-    if attempts >= tonumber(ARGV[8]) and fails_w / attempts >= tonumber(ARGV[9]) then
-        open_via_rate = true
+    if attempts >= tonumber(ARGV[8]) then
+        if fails_w / attempts >= tonumber(ARGV[9]) then
+            open_via_rate = true
+        else
+            -- 窗口证据充足且健康: 连败是噪声,抑制连续通道(迭代 6)
+            window_healthy = true
+        end
     end
 end
 
@@ -197,7 +203,8 @@ if force_open == 1 or is_probe == 1 then
 else
     failures = failures + 1
 end
-if force_open == 1 or is_probe == 1 or open_via_rate or failures >= threshold then
+local open_via_streak = failures >= threshold and not window_healthy
+if force_open == 1 or is_probe == 1 or open_via_rate or open_via_streak then
     -- 递增 streak 的只有率通道开路与探针失败重开(C1: 连续通道/force_open 不递增)
     if open_via_rate or is_probe == 1 then
         redis.call('HINCRBY', KEYS[1], 'reopen_streak', 1)