The router silently extended every request to all available vendors regardless
of config, so an explicit single-vendor choice still fell back to others and
returned data from an unexpected source (#988, #289), and serious primary-vendor
errors were swallowed without a trace (#989). The configured vendor list is now
the exact chain (list several for ordered fallback; "default" uses all), unknown
vendors raise, and swallowed vendor errors are logged. Adds an autouse config
isolation fixture so vendor config can't leak between tests.