Move push rule models under PushRules:: namespace

Summary

The PushRules bounded context is registered in config/bounded_contexts.yml, but none of the push rule models have been moved under it yet. All push rule models are currently top-level classes with rubocop todo entries to suppress Gitlab/NamespacedClass and Gitlab/BoundedContexts violations.

Models to move

• PushRule → PushRules::PushRule
• GroupPushRule → PushRules::GroupPushRule
• OrganizationPushRule → PushRules::OrganizationPushRule
• ProjectPushRule → PushRules::ProjectPushRule
• PushRuleFinder → PushRules::PushRuleFinder
• GroupPushRuleFinder → PushRules::GroupPushRuleFinder
• ProjectPushRuleFinder → PushRules::ProjectPushRuleFinder

Tasks

• Move models and finders under PushRules:: namespace
• Update all references across the codebase
• Remove entries from .rubocop_todo/gitlab/namespaced_class.yml
• Remove entries from .rubocop_todo/gitlab/bounded_contexts.yml

References

• Bounded context definition: config/bounded_contexts.yml
• Project push rule implementation plan: #499156