Hosted Data API can keep non-existent public/api schemas in PostgREST db-schemas after dashboard exposed-schema changes

# Bug report ## Describe the bug In a hosted Supabase project, the Dashboard `Integrations > Data API > Settings > Exposed schemas` value can appear to be updated to custom schemas only, while the running PostgREST process still tries to build its schema cache with non-existent schemas such as `public` or a previously exposed custom schema. This breaks the Data API entirely because PostgREST fails schema-cache loading with SQLSTATE `3F000` before serving RPC requests. This looks related to, but different from, issues such as #40617 and #43723. In this case the project intentionally does not have a `public` schema, the Dashboard Data API settings were saved repeatedly with only custom schemas, and toggling the Data API off/on did not reliably clear the runtime `db-schemas` value. ## Sanitized PostgREST logs Example after removing a legacy `api` schema from exposed schemas and saving the Dashboard Data API settings: ```json { "event_message": "Failed to load the schema cache using db-schemas=api,agent_api,ai_api,auth_api,human_api,machine_api and db-extra-search-path=extensions. {\"code\":\"3F000\",\"details\":null,\"hint\":null,\"message\":\"schema \\\"api\\\" does not exist\"}" } ``` Example after resetting/toggling the Data API settings in the Dashboard: ```json { "event_message": "Failed to load the schema cache using db-schemas=public,auth_api,human_api and db-extra-search-path=extensions. {\"code\":\"3F000\",\"details\":null,\"hint\":null,\"message\":\"schema \\\"public\\\" does not exist\"}" } ``` All project refs, hostnames, request ids, and event ids have been removed from these examples. ## Database state A read-only SQL check showed that only the custom API schemas existed and both `api` and `public` did not exist: ```sql select schema_name, to_regnamespace(schema_name) is not null as schema_exists from ( values ('api'), ('public'), ('agent_api'), ('ai_api'), ('auth_api'), ('human_api'), ('machine_api') ) as target(sche…