Refactor test for user detail sanitization

What does this MR do and why?

Refactors the UserDetail model's sanitize_attrs method and its corresponding tests for improved code organization and maintainability.

Key changes:

1. Move sanitize_attrs to private section: The method was previously declared as public but is only called internally via before_validation callback. Moving it to the private section better reflects its intended usage.

2. Refactor tests to use parameterized table syntax: The #sanitize_attrs describe block in spec/models/user_detail_spec.rb has been refactored from shared examples with loops to use RSpec's parameterized table syntax (where/with_them), making the tests more readable and easier to extend.

Behavioral changes:

None. This is a pure refactoring with no changes to runtime behavior. The sanitization logic remains exactly the same:

• HTML tags are stripped from all fields
• Script and iframe elements are removed
• Ampersands (&) are encoded to & for linkedin, twitter, and website_url fields
• Ampersands are preserved (not encoded) for location and organization fields

References

• Related to issue #577060 (closed)
• Sanitizable concern: app/models/concerns/sanitizable.rb

Screenshots or screen recordings

N/A - Backend refactoring with no UI changes

How to set up and validate locally

1. Run the test suite to verify the refactored tests pass:

   bundle exec rspec spec/models/user_detail_spec.rb -e "sanitize_attrs"

2. Verify sanitization still works correctly in Rails console:

   user = User.first
   ud = user.user_detail
   
   # HTML tags should be stripped
   ud.linkedin = '<script>alert("xss")</script>test'
   ud.save
   ud.linkedin # Should output: 'test'
   
   # Ampersand handling for linkedin/twitter/website_url (encodes)
   ud.twitter = 'test&value'
   ud.save
   ud.twitter # Should output: 'test&amp;value'
   
   # Ampersand handling for location/organization (preserves)
   ud.location = 'San Francisco & Bay Area'
   ud.save
   ud.location # Should output: 'San Francisco & Bay Area'

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug - Tests refactored using parameterized table syntax
• Conforms to the code review guidelines
• Conforms to the style guides
• All RSpec tests pass
• RuboCop linting passes with no offenses
• No database migrations required (model-only changes)