Investigate what actions change "Date modified" for end user publicly-visible forms [key actions]

## Description
As per [#incident-2025-06-20-investigating-validation-issue](https://gcdigital.slack.com/archives/C092D5RD5LL)

**Look into what actions taken on the form impact the “updated_at” value on the publicly visible form**
- We assumed the "date modified" was the "date published" but this was not the case
- Perhaps the "date published" would be a more accurate and useful thing to post
- Should only include updates that affect the public, not just any update


**Related**

#3991
#5545

Start by reviewing the public-facing form behavior described here and the related issues #3991 and #5545, then trace how updated_at and the date published value are exposed. Document which key actions change the publicly visible date, distinguishing public-impacting updates from internal changes; done means the behavior and any recommended date are clearly recorded.

Written by the indexing model from the issue text.