Previously, WebSearchTool cached the API key in __init__, so keys added
to config.json or env vars after gateway startup were never picked up.
This caused a confusing 'BRAVE_API_KEY not configured' error even after
the key was correctly set (issue #1069).
Changes:
- Store the init-time key separately, resolve via property at each call
- Improve error message to guide users toward the correct fix
Closes#1069
