fix: add warning log and clearer error message for missing Tavily API key

Log a warning at startup when TAVILY_API_KEY is not configured, and
return a more actionable error message when search is invoked without it.

Made-with: Cursor

Author: wuxiaohan10

backend/app/core/tools/buildin/research_tools.py

@@ -4,11 +4,15 @@
 using Tavily for URL discovery and fetching full webpage content.
 """
 
+import logging
+
 import httpx
 from langchain_core.tools import InjectedToolArg, tool
 from markdownify import markdownify
 from typing_extensions import Annotated, Literal
 
+logger = logging.getLogger(__name__)
+
 try:
     from tavily import TavilyClient
 except ImportError:
@@ -18,6 +22,10 @@
 try:
     tavily_client = TavilyClient() if TavilyClient else None
 except Exception:
+    logger.warning(
+        "Tavily client initialization failed. "
+        "Please set the TAVILY_API_KEY environment variable to enable web search."
+    )
     tavily_client = None
 
 
@@ -62,7 +70,10 @@ def tavily_search(
         Formatted search results with full webpage content
     """
     if tavily_client is None:
-        return "Error: Tavily client is not available. Please install tavily-python package."
+        return (
+            "Error: Tavily client is not available. "
+            "Please set the TAVILY_API_KEY environment variable and restart the service."
+        )
 
     # Use Tavily to discover URLs
     search_results = tavily_client.search(