fix(executor): prevent streaming from 429 response when fallback available

Fix critical bug where ExecuteStream would create a streaming channel
using a 429 error response instead of continuing to the next fallback
model after exhausting retries.

When 429 retries were exhausted and a fallback model was available,
the inner retry loop would break but immediately fall through to the
streaming channel creation, attempting to stream from the failed 429
response instead of trying the next model.

Solution: Add shouldContinueToNextModel flag to explicitly skip the
streaming logic and continue the outer model loop when appropriate.

Identified by: codex-bot review
Ref: https://github.com/router-for-me/CLIProxyAPI/pull/280#pullrequestreview-3484479106

internal/runtime/executor/gemini_cli_executor.go

@@ -279,6 +279,7 @@ func (e *GeminiCLIExecutor) ExecuteStream(ctx context.Context, auth *cliproxyaut
 		var httpResp *http.Response
 		var payload []byte
 		var errDo error
+		shouldContinueToNextModel := false
 
 		// Inner retry loop for 429 errors on the same model
 		for retryCount := 0; retryCount <= maxRetries; retryCount++ {
@@ -364,6 +365,7 @@ func (e *GeminiCLIExecutor) ExecuteStream(ctx context.Context, auth *cliproxyaut
 						// Exhausted retries for this model, try next model if available
 						if idx+1 < len(models) {
 							log.Infof("gemini cli executor: rate limited, exhausted %d retries for stream model %s, trying fallback model: %s", maxRetries, attemptModel, models[idx+1])
+							shouldContinueToNextModel = true
 							break // Break inner loop to try next model
 						} else {
 							log.Infof("gemini cli executor: rate limited, exhausted %d retries for stream model %s, no additional fallback model", maxRetries, attemptModel)
@@ -385,6 +387,11 @@ func (e *GeminiCLIExecutor) ExecuteStream(ctx context.Context, auth *cliproxyaut
 			break
 		}
 
+		// If we need to try the next fallback model, skip streaming logic
+		if shouldContinueToNextModel {
+			continue
+		}
+
 		out := make(chan cliproxyexecutor.StreamChunk)
 		stream = out
 		go func(resp *http.Response, reqBody []byte, attempt string) {