Address code review comments from PR #2551

[mroderick]

The following comments were left on PR #2551 by @olleolleolle:

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Comment 1: Test organization

olleolleolle Apr 9, 2026

Ergonomics: I assume it'd be possible to have a `describe "handling deduplication"` block, wrapping these test cases, and placing them in a spec/models/invitation_manager_spec.rb, so that "flipping between test and implementation" works in an expected way.

Suggested action: Move the deduplication tests from `spec/models/invitation_manager_deduplication_spec.rb` into `spec/models/invitation_manager_spec.rb` under a `describe "handling deduplication"` block.

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Comment 2: Use `described_class` instead of hardcoded class name

olleolleolle Apr 9, 2026

Suggested change:
```ruby
subject(:manager) { InvitationManager.new }
```
to:
```ruby
subject(:manager) { described_class.new }
```

Very minor in this test file, but in other cases, when it's repeated a lot in a test, it makes renaming classes more cumbersome.

Suggested action: Update the test file to use `described_class.new` instead of `InvitationManager.new`.