* fix(agent-core-v2): support services.moonshot_search config for WebSearch
The v2 engine only resolved the WebSearch backend from the managed Kimi OAuth provider, so an environment that configures web search purely via [services.moonshot_search] (api key, no login) got no provider and the WebSearch tool stayed hidden. Register the services config section and resolve the provider config-first, mirroring v1 precedence.
* fix(agent-core-v2): preserve custom services config