feat: add frontend-app-enterprise-checkout to transifex config

[ssrijaindukuri-sonata]

Summary

Part of ENT-11885 ([FE] internationalize
frontend-app-enterprise-checkout). Registers frontend-app-enterprise-checkout with Transifex so its
source strings start being tracked and translated.

Without this entry, Transifex has no resource configured for this app — extracted strings from
transifex_input.json would have nowhere to be pushed to, and there'd be no path for translated
<lang>.json files to be pulled back into via Atlas.

Changes

• transifex.yml: added a frontend-app-enterprise-checkout entry, alphabetized with the rest of
  the frontend-app-* block, following the same schema (filter_type: file, KEYVALUEJSON,
  mode: onlyreviewed) used by every neighboring MFE.
  • source_file: translations/frontend-app-enterprise-checkout/src/i18n/transifex_input.json
  • translation_files_expression: translations/frontend-app-enterprise-checkout/src/i18n/messages/<lang>.json

This path matches exactly what the app's Makefile already expects in its atlas pull mapping
(see frontend-app-enterprise-checkout#228).

Related

• frontend-app-enterprise-checkout#228 — fixes an extraction bug and adds Atlas Makefile hooks; should merge before this.
• edx-internal#14785 — enables the Atlas pull in CI; should merge after this.

[github-actions]

✅ All .json translation files are valid.

This comment has been posted by the validate-translation-files.yml GitHub Actions workflow.

[github-actions]

✅ All .po translation files are valid.

This comment has been posted by the validate-translation-files.yml GitHub Actions workflow.

[Copilot]

Pull request overview

Adds a new Transifex resource entry for frontend-app-enterprise-checkout so that this repository can track and manage its JSON translation files via the existing transifex.yml configuration.

Changes:

• Add a KEYVALUEJSON Transifex mapping for frontend-app-enterprise-checkout (source file + translated messages path).

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[ssrijaindukuri-sonata]

This is expected for a new-repo bootstrap entry, not a bug — source_file and
translation_files_expression aren't required to exist in this repo yet. transifex_input.json
gets generated and pushed by frontend-app-enterprise-checkout's own CI when it runs
make extract_translations; the messages/<lang>.json files get written back here by the
Transifex sync job once translations are reviewed. Every other frontend-app-* entry in this
file followed the same pattern when it was first onboarded, so this won't break sync/CI.

The one real prerequisite — making sure the source strings actually extract cleanly — is handled
in a companion PR: frontend-app-enterprise-checkout#228 fixes a defineMessages bug that was
silently hiding two strings from the extractor. That should merge before or alongside this one;
this config itself is safe to merge on its own.