AO3-4122 Prevent duplicate translation languages for admin posts

[pmonfort]

Pull Request Checklist

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Issue

https://otwarchive.atlassian.net/browse/AO3-4122

Purpose

Adds a validation so an admin post can't be saved as a translation of another post when that post already has a translation in the same language. The error is reported on the translated post field, alongside the existing "cannot be same language as original post" and "cannot be posted before the original post" errors.

Testing Instructions

1. Log in as an admin with the translation role.
2. Post a news post in English.
3. Go to Post AO3 News, choose a language (e.g. Deutsch), ID in "Translation of", and post it.
4. Repeat step 3 with the same language and the same "Tran
5. Expected: the post is not saved and the error "Translated post already has a translation in this language" is shown.
6. Repeat step 3 with a different language: the translation is saved.
7. Edit the translation from step 3 (change the title only) and save: no error.

Credit

Pablo Monfort (he/him)

[pmonfort]

Disabled this cop instead of adding a unique index on (translated_post_id, language_id). Production likely has duplicates already (that's what this issue is about), so the index would need a data cleanup first, and that felt out of scope here. The validation itself runs the same query either way and uses the existing index on translated_post_id.

[pmonfort]

The issue only mentions published translations, but this implementation also blocks the case where the existing translation is still a draft. Drafts are shared between all admins, otherwise a second translation in the same language could be posted while the draft is pending.