Refactor apply trials action to use service

What does this MR do and why?

Iteratively working towards !115567 (merged)

📓 changes covered by ee/spec/features/trials/saas/ tests

• refactor apply trials action to use service, moving business logic out of the controller layer.
• fine tune create_lead action:
  • update the reason name when there is no_single_namespace found for clarity
  • remove not possible condition of not_found
• rename trial eligible related method names around the namespace wording as that is where the scope is defined.
• clarify the namespace assignment in the service layer and be explicit about the use.

Screenshots or screen recordings

no change.

How to set up and validate locally

note basically trying to emulate feature spec steps in ee/spec/features/trials/saas/*

1. Setup to simulate SaaS and restart GDK
2. Create 2 new groups.
3. Visit trials new path as in http://gdk.test:3000/-/trials/new.
4. Fill out the form...
5. The rest would be really hard to emulate here w/o a fully setup local customersDot and is why we have the feature specs that haven't needed to change with this MR.

MR acceptance checklist

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Related to #393969 (closed)