fix(proxy): wire AppProxyConfig.max_retries into request forwarder

The UI has exposed "请求失败时的重试次数 (0-10, default 3)" since the
auto-failover panel was added, but the value was silently dropped —
RequestForwarder never received it and the per-provider loop walked the
whole list regardless. From the user's perspective the setting was
inert.

Thread AppProxyConfig.max_retries through create_forwarder into
RequestForwarder, derive max_attempts = max_retries + 1 (so max_retries=0
matches the UI copy "0 retries" = single attempt), and break the loop
once attempts hit the cap. The check is placed before the circuit
breaker allow-permit so an over-cap iteration does not waste a HalfOpen
probe slot.

When auto-failover is disabled we also force max_retries to 0, mirroring
how timeouts already bypass in that mode — "no failover" should mean
"one provider, one try", not "limited retries against the same list".

src-tauri/src/proxy/handler_context.rs

@@ -211,6 +211,13 @@ impl RequestContext {
                 (0, 0, 0)
             };
 
+        // 故障转移关闭时强制 max_retries=0（仅尝试 1 个 provider），与「不超时 + 不切换」语义一致。
+        let max_retries = if self.app_config.auto_failover_enabled {
+            self.app_config.max_retries
+        } else {
+            0
+        };
+
         RequestForwarder::new(
             state.provider_router.clone(),
             non_streaming_timeout,
@@ -227,6 +234,7 @@ impl RequestContext {
             self.rectifier_config.clone(),
             self.optimizer_config.clone(),
             self.copilot_optimizer_config.clone(),
+            max_retries,
         )
     }