perf: add HTTP connection pooling across providers and disable proxy buffering on SSE streams

internal/providers/deepseek.go

@@ -22,7 +22,7 @@ type DeepSeekProvider struct {
 
 func NewDeepSeekProvider(cfg config.DeepSeekConfig, apiKey string) *DeepSeekProvider {
 	return &DeepSeekProvider{
-		client: resty.New().SetTimeout(10 * time.Minute),
+		client: NewOptimizedRestyClient(10 * time.Minute),
 		config: cfg,
 		apiKey: apiKey,
 	}