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

internal/providers/gemini.go

@@ -21,7 +21,7 @@ type GeminiProvider struct {
 
 func NewGeminiProvider(cfg config.GeminiConfig, apiKey string) *GeminiProvider {
 	return &GeminiProvider{
-		client: resty.New().SetTimeout(10 * time.Minute),
+		client: NewOptimizedRestyClient(10 * time.Minute),
 		config: cfg,
 		apiKey: apiKey,
 	}