Only mock ci connections in single db mode

What does this MR do and why?

Changes mocked_ci_connection to not setup the mocked ci connection when a real ci database is configured. This prevents an issue where calling remove_connection clears the connection_specification_name for Ci::ApplicationRecord, so when it reconnects, it picks uses the default specification name of ActiveRecord::Base, causing it to use the default connection as well.

This was causing flaky test failures on the decomposed pipeline, when running these specs in this order:

rspec spec/lib/gitlab/database/query_analyzers/gitlab_schemas_metrics_spec.rb spec/models/acts_as_taggable_on/tagging_spec.rb

How to set up and validate locally

1. Verify that this command fails on master:

   GITLAB_LOAD_BALANCING_REUSE_PRIMARY_ci=main GITLAB_USE_MODEL_LOAD_BALANCING=true bundle exec rspec spec/lib/gitlab/database/query_analyzers/gitlab_schemas_metrics_spec.rb spec/models/acts_as_taggable_on/tagging_spec.rb

   with an error like:

     1) ActsAsTaggableOn::Tagging has the same connection as Ci::ApplicationRecord
     Failure/Error: expect(described_class.retrieve_connection.execute(query).first).to eq(Ci::ApplicationRecord.retrieve_connection.execute(query).first)
       expected: {"current_database"=>"gitlabhq_test"}
            got: {"current_database"=>"gitlabhq_test_ci"}

2. Run the same test above with the change from this branch, and it should pass.

MR acceptance checklist

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