feat(registry/runtime): add Gemini 2.5 model and increase buffer sizes

- Added new "Gemini 2.5 Flash Image Preview" model definition, with enhanced image generation capabilities.
- Increased scanner buffer size to 20,971,520 bytes across executors and translators to handle larger payloads.

internal/runtime/executor/iflow_executor.go

@@ -141,8 +141,8 @@ func (e *IFlowExecutor) ExecuteStream(ctx context.Context, auth *cliproxyauth.Au
 		defer func() { _ = resp.Body.Close() }()
 
 		scanner := bufio.NewScanner(resp.Body)
-		buf := make([]byte, 1024*1024)
-		scanner.Buffer(buf, 1024*1024)
+		buf := make([]byte, 20_971_520)
+		scanner.Buffer(buf, 20_971_520)
 		var param any
 		for scanner.Scan() {
 			line := scanner.Bytes()