Function: resolveWebSearchConfig()
resolveWebSearchConfig(
config?):ResolvedWebSearchConfig
Resolve plugin config over environment defaults into the concrete
ResolvedWebSearchConfig. Which model is used stays lazy - it is
picked at call time against the live catalogue - but everything else is
settled here, and a contradiction (unparseable WEB_SEARCH_TOOLS, an
unknown URL policy, a policy that disagrees with its allow-list) throws a
ConfigurationError naming the field and the environment variable.