3613737: Add ContentSingletonRouteContext to expose entity:content_singleton context

Register a ContentSingletonRouteContext context provider that reads the content_singleton route parameter and exposes it as a non-required entity:content_singleton context with a route cache context.

This mirrors the NodeRouteContext pattern from Drupal core and fixes condition plugins (e.g. entity_bundle:content_singleton) that require an entity:content_singleton context to resolve — previously they would always evaluate FALSE because no matching context was available.

Also registers the service with the context_provider tag and adds kernel test coverage for all runtime branches.

Closes #3613737