Fix incorrect token type in suggested reviewers registration

What does this MR do and why?

This MR fixes an invalid token type that is used in the suggested reviewers' project registration endpoint. We need to extract the actual token value from the response, not the PersonalAccessToken object.

The following error can be observed in the log.

Invalid argument for string field 'access_token' (given PersonalAccessToken).

This bug is introduced in !101221 (merged).

The client-facing feature is behind a feature flag suggested_reviewers_control and is disabled by default.

Screenshots or screen recordings

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How to set up and validate locally

1. Set ultimate license on a group http://gdk.test:3000/admin/groups
2. Create a project in the ultimate group or use an existing one, e.g. http://gdk.test:3000/gitlab-org/gitlab-test
3. Start rails console bundle exec rails c
4. Enable SAAS simulation

   ENV['GITLAB_SIMULATE_SAAS'] = '1'

5. Set the feature flag

   project = Project.find(2)
   Feature.enable(:suggested_reviewers_control, project)

6. Enable the suggested reviewers setting and enable to access token creation setting

   project.project_setting.update(suggested_reviewers_enabled: true)
   project.namespace.namespace_settings.update(resource_access_token_creation_allowed: true)
   project.reload

7. Add a fake secret

   ENV['SUGGESTED_REVIEWERS_SECRET'] = 'FAKEefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901'

8. Invoke the service and observe the response object

   user = User.first
   service = ::Projects::RegisterSuggestedReviewersProjectService.new(project: project, current_user: user)
   response = service.execute
   
   # Before - fail due to type error
   Google::Protobuf::TypeError: Invalid argument for string field 'access_token' (given PersonalAccessToken).
   
   # After - fail due to service is not available, good enough to validate the previous error has been fixed
   #<ServiceResponse:0x000000013dab78c0 @http_status=nil, @message="Failed to register project", @payload={}, @reason=:client_request_failed, @status=:error>

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #377917