Migrate snippet service subclasses to BaseProjectServices [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

What

• Continuation of refactoring discussed in !30979 (comment 337747596), and continued in previous MR !59182 (merged) for issuables.
• Refactors Snippet-related service classes to subclass BaseContainerService instead of BaseService, via Projects::BaseProjectService.

Not in scope

• Refactoring PersonalSnippet services to not be a subclass of BaseContainerService, because it does not involve a project. We could potentially split `BaseContainerService into modules in a separate refactor. We could open an issue, but this will likely need to be holistically addressed for many service classes, perhaps as part of https://gitlab.com/gitlab-org/architecture/tasks/-/issues/7

Why

This provides a greater degree of type safety in order to more easily and safely introduce a new named argument spam_params to support !58603 (merged).

The additional type safety will also make future refactors easier and less risky, such as https://gitlab.com/gitlab-org/architecture/tasks/-/issues/7 and !54792 (merged).

Screenshots (strongly suggested)

Refactored class hierarchy

Expand to see class hierarchy changed, or view screenshot below:

Summary of class hierarchy changed

Snippets::BaseService (base_service.rb)
    Snippets::CreateService (create_service.rb)
    Snippets::UpdateService (update_service.rb)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because it is a pure refactor, there are no user facing changes.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

See sections above for a detailed discussion of the risk.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team