fix(auth): free trial duplicate reminder emails

[StaberindeZA]

Because

• Free trial reminder emails are sending duplicate emails.

This pull request

• Update call of alreadySentEmail, by free trial logic, to pass in required current_period_start instead of current_period_end.

Issue that this pull request solves

Closes: #PAY-3652

Checklist

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• If applicable, I have modified or added tests which pass locally.
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• I have verified that my changes render correctly in RTL (if appropriate).
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How to review (Optional)

• Subscribe to a free trial
• Run the reminder script with the following parameters. Note the reminder length might need to change depending on trial reminder.

CONFIG_FILES='config/secrets.json' NODE_ENV=dev FIRESTORE_EMULATOR_HOST=localhost:9090 yarn workspace fxa-auth-server subscription-reminders -t true --free-trial-ending-reminder-length=13

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[Copilot]

Pull request overview

Fixes duplicate free-trial reminder emails by aligning the “already sent” de-duplication check with the subscription’s current billing/trial cycle start time.

Changes:

• Pass subscription.current_period_start (ms) into alreadySentEmail for freeTrialEndingReminder instead of current_period_end.
• Minor formatting change for freeTrialEndingReminderEnabled assignment.

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[Copilot]

This change fixes the de-dupe key for free-trial reminders, but there’s no regression test covering sendFreeTrialEndingReminderEmail/sendFreeTrialEndingReminders to ensure it de-dupes within the same current_period_start and doesn’t send duplicates on repeated runs. Please add a unit test (in subscription-reminders.spec.ts) that sets up a trialing subscription, stubs SentEmail.findLatestSentEmailByType with sentAt after the period start, and asserts the method returns false (and/or that alreadySentEmail is invoked with current_period_start).

[xlisachan]

Thanks Reino!