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/config/configuration.py

@@ -54,6 +54,9 @@ class Configuration:
     enforce_web_search: bool = (
         False  # Enforce at least one web search step in every plan
     )
+    enforce_researcher_search: bool = (
+        True  # Enforce that researcher must use web search tool at least once
+    )
     interrupt_before_tools: list[str] = field(
         default_factory=list
     )  # List of tool names to interrupt before execution