[web] email content previews for notification emails (#34576)

* add content previews to notification emails
* add fetcher for doc lines
* gate content previews behind a split test
* make composable blocks for rendering

GitOrigin-RevId: c8df384d55a301084b98df76fd3edd6dde6ea50c
This commit is contained in:
Kristina
2026-06-22 08:06:25 +00:00
committed by Copybot
parent 63a605484c
commit a53b164b45
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