* fix: ensure researcher agent uses web search tool instead of generating URLs (#702)
- Add enforce_researcher_search configuration option (default: True) to control web search requirement
- Strengthen researcher prompts in both English and Chinese with explicit instructions to use web_search tool
- Implement validate_web_search_usage function to detect if web search tool was used during research
- Add validation logic that warns when researcher doesn't use web search tool
- Enhance logging for web search tools with special markers for easy tracking
- Skip validation during unit tests to avoid test failures
- Update _execute_agent_step to accept config parameter for proper configuration access
This addresses issue #702 where the researcher agent was generating URLs on its own instead of using the web search tool.
* fix: addressed the code review comment
* fix the unit test error and update the code
