### What problem does this PR solve?
issue:
[#10417](https://github.com/infiniflow/ragflow/issues/10417)
change:
Adjusted the `searxng_url` priority logic to ensure the
frontend-provided URL takes precedence over the model’s default
configuration. This allows user-specified SearXNG endpoints to be
correctly applied during execution, improving flexibility across
different environments.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)