fix(mcp): deny all tools on invalid credentials in search filtering

Split the combined except clause in _tool_allowed_for_current_user so
that PermissionError (bad API key) returns False for every tool,
matching RBACToolVisibilityMiddleware's fail-closed behaviour.
ValueError (no auth source configured) retains the existing public-tool
fallback. Adds a test to cover the new deny-all path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>