Question 'cause I've run into this. If the text is more than fits in the alt-text box, I assume summarizing slightly is preferred to something like "..." or "continues?"

No. Put in dummy images to include the whole thing if necessary.

The latest update of Bluesky doubled the character space for alt text. It should be less of a problem now. 🙏🏼 I do the best I can. If it's a couple of sentences and there's a URL so someone can go read it, I write [truncated for space]. Idk "rules." I do occasionally summarize.