RegistriesResolver should return null when feature flag is disabled

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What does this MR do and why?

This MR adds a check in RegistriesResolver#resolve. It now checks replicator_class.replication_enabled? and returns none if the corresponding feature flag is disabled.

Added a test context context 'when replication is disabled' dothat stub the replication flag to be false. The example inside of that context expects the result resolve_registries to be empty.

Global geo setting is now stubbed to enabled: true for all test in the shared examples to ensure all Geo registry resolvers behave consistently.

References

Related to #377988 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup a development environment with 2 GDKs

2. Run the specs for any of the resolvers that use the shared examples in /ee/spec/support/shared_examples/graphql/geo/geo_registries_resolver_shared_examples.rb, for example:

   bin/rspec ee/spec/graphql/resolvers/geo/container_repository_registries_resolver_spec.rb

3. All spec should pass

MR acceptance checklist

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Related to #377988 (closed)