feat(data-export): Improve observability for failure emails

[sentry]

This PR enhances the observability of data export failure emails by adding explicit logging at key stages of the email_failure method in sentry/data_export/models.py.

Previously, if a data export failed and the subsequent failure email was not dispatched (e.g., due to a missing user ID, no associated email, or issues within the notification service itself), there was no clear log indicating why the email was not sent or if it was attempted at all. This made debugging user reports of not receiving failure notifications challenging.

Changes include:

• Added logger.warning statements for early exits in email_failure when user_id is None or user.email is not found. This provides visibility into cases where an email cannot be sent due to missing user information.
• Introduced a data-export-failure.mail.sent logger.info statement after both the NotificationService.notify_async and MessageBuilder.send_async paths. This log confirms that an attempt to send the failure email was made and indicates which notification mechanism was used via a via tag.

These changes will provide clearer telemetry for data export failure notifications, making it easier to diagnose why users might not be receiving them.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[JoshuaKGoldberg]

Context: I directed Sentry to make this to help investigate other data export failures.

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀