fix: ensure researcher agent uses web search tool instead of generating URLs (#702) (#704)

* 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

src/prompts/researcher.md

@@ -37,7 +37,8 @@ You have access to two types of tools:
 3. **Plan the Solution**: Determine the best approach to solve the problem using the available tools.
 4. **Execute the Solution**:
    - Forget your previous knowledge, so you **should leverage the tools** to retrieve the information.
-   - Use the {% if resources %}**local_search_tool** or{% endif %}**web_search** or other suitable search tool to perform a search with the provided keywords.
+   - **CRITICAL**: You MUST use the {% if resources %}**local_search_tool** or{% endif %}**web_search** tool to search for information. NEVER generate URLs on your own. All URLs must come from tool results.
+   - **MANDATORY**: Always perform at least one web search using the **web_search** tool at the beginning of your research. This is not optional.
    - When the task includes time range requirements:
      - Incorporate appropriate time-based search parameters in your queries (e.g., "after:2020", "before:2023", or specific date ranges)
      - Ensure search results respect the specified time constraints.
@@ -71,6 +72,8 @@ You have access to two types of tools:
 
 # Notes
 
+- **CRITICAL**: NEVER generate URLs on your own. All URLs must come from search tool results. This is a mandatory requirement.
+- **MANDATORY**: Always start with a web search. Do not rely on your internal knowledge.
 - Always verify the relevance and credibility of the information gathered.
 - If no URL is provided, focus solely on the search results.
 - Never do any math or any file operations.