Respect config layers when removing plugin marketplaces (#40683)

## Why

Removing a base-user marketplace must not delete a snapshot that is still
referenced by another enabled configuration layer.

## What changed

- Load the current configuration stack for marketplace commands in the CLI and
  app server.
- Reject removal when a matching marketplace is defined by another enabled
  layer, and identify the configuration source that must be updated instead.
- Allow cleanup of an installed marketplace snapshot even when no base-user
  entry exists.

## Testing

Cover project, enterprise-managed, system, and session-flag configurations, as
well as snapshot-only removal through the CLI and app server.

GitOrigin-RevId: 4f0853a40c2b5fae0410696b79eee4284239c7a9