When the Brave Search API returns HTTP 200 with zero results, log a
warning with the response body preview to help diagnose silent failures.
This addresses cases where the API response format has changed or a
non-standard error response is misinterpreted as a successful empty
result, leaving the LLM with a misleading "No results" message.
Previously, empty results were silently returned to the LLM as "No
results for: <query>", making it impossible to distinguish between
genuinely empty results and API format mismatches or silent errors.
Closes#3125
b2fbe83565