fix(proxy): resolve circuit breaker state persistence and HalfOpen deadlock

This commit addresses several critical issues in the failover system:

**Circuit breaker state persistence (previous fix)**
- Promote ProviderRouter to ProxyState for cross-request state sharing
- Remove redundant router.rs module
- Fix 429 errors to be retryable (rate limiting should try other providers)

**Hot-update circuit breaker config**
- Add update_circuit_breaker_configs() to ProxyServer and ProxyService
- Connect update_circuit_breaker_config command to running circuit breakers
- Add reset_provider_circuit_breaker() for manual breaker reset

**Fix HalfOpen deadlock bug**
- Change half_open_requests from cumulative count to in-flight count
- Release quota in record_success()/record_failure() when in HalfOpen state
- Prevents permanent deadlock when success_threshold > 1

**Fix duplicate select_providers() call**
- Store providers list in RequestContext, pass to forward_with_retry()
- Avoid consuming HalfOpen quota twice per request
- Single call to select_providers() per request lifecycle

**Add per-provider retry with exponential backoff**
- Implement forward_with_provider_retry() with configurable max_retries
- Backoff delays: 100ms, 200ms, 400ms, etc.

src-tauri/src/commands/proxy.rs

@@ -130,15 +130,17 @@ pub async fn reset_circuit_breaker(
     provider_id: String,
     app_type: String,
 ) -> Result<(), String> {
-    // 重置数据库健康状态
+    // 1. 重置数据库健康状态
     let db = &state.db;
     db.update_provider_health(&provider_id, &app_type, true, None)
         .await
         .map_err(|e| e.to_string())?;
 
-    // 注意：熔断器状态在内存中，重启代理服务器后会重置
-    // 如果代理服务器正在运行，需要通知它重置熔断器
-    // 目前先通过数据库重置健康状态，熔断器会在下次超时后自动尝试半开
+    // 2. 如果代理正在运行，重置内存中的熔断器状态
+    state
+        .proxy_service
+        .reset_provider_circuit_breaker(&provider_id, &app_type)
+        .await?;
 
     Ok(())
 }
@@ -161,9 +163,19 @@ pub async fn update_circuit_breaker_config(
     config: CircuitBreakerConfig,
 ) -> Result<(), String> {
     let db = &state.db;
+
+    // 1. 更新数据库配置
     db.update_circuit_breaker_config(&config)
         .await
-        .map_err(|e| e.to_string())
+        .map_err(|e| e.to_string())?;
+
+    // 2. 如果代理正在运行，热更新内存中的熔断器配置
+    state
+        .proxy_service
+        .update_circuit_breaker_configs(config)
+        .await?;
+
+    Ok(())
 }
 
 /// 获取熔断器统计信息（仅当代理服务器运行时）