[lb] Fix site validation on GetCourseOptionFromCodes service

[elceebee]

Context

I'm really surprised this hasn't come up before -- This is a bug in the GetCourseOptionFromCodes service used when making an offer and changing courses from the API. But it was raised in Sandbox, support ticket.

Changes proposed in this pull request

Call uniq, so courses with full and part-time options don't give false invalid errors.

Guidance to review

In sandbox

get_course_option_service = GetCourseOptionFromCode.new(
  provider_code: 'E42',
  course_code: '2PPM' # this course has full and part time options for main site
  study_mode: 'part_time',
  site_code: '-' # Main site,
  recruitment_cycle_year: 2026,
)

get_course_option_service.valid? will be false.

This is because

provider = Provider.find_by(code: 'E42')
course = provider.courses.find_by(recruitment_cycle_year: 2026, code: '2PPM')
sites = provider.sites
  .joins(:course_options) 
  .merge(CourseOption.selectable.where(course:)
  .where(code: '-')
  .count 

=> 2

But the same with uniq.count is 1.

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card
• This code adds a column or table to the database
  • This code does not rely on migrations in the same Pull Request
  • decide whether it needs to be in analytics yml file or analytics blocklist
  • data insights team has been informed of the change and have updated the pipeline
  • the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated
  • does the code safely backfill existing records for consistency

[JamieCleare2525]

LGTM!