fix(vision): log error when vision client is unavailable

Previously the early return for unconfigured vision model was silent.
Now logs an error so the failure is visible in logs for debugging.

Inspired by PR #839 by aydnOktay.

Co-authored-by: aydnOktay <aydnOktay@users.noreply.github.com>

tools/vision_tools.py

@@ -259,6 +259,7 @@ async def vision_analyze_tool(
         
         # Check auxiliary vision client availability
         if _aux_async_client is None or DEFAULT_VISION_MODEL is None:
+            logger.error("Vision analysis unavailable: no auxiliary vision model configured")
             return json.dumps({
                 "success": False,
                 "analysis": "Vision analysis unavailable: no auxiliary vision model configured. "