[6.x] Ensure fields into the first section of a tab instead of the last

[mynetx]

When a blueprint tab has multiple sections, fields added at runtime via ensureField() get appended to the last section. The most visible casualty is the Super User toggle: add a new section to the user blueprint and the toggle jumps into it, away from the other user details.

Before the v4 tabs rewrite (#7746), ensured fields defaulted to the first section. This restores that behaviour: ensured fields that don't already exist in the blueprint now land at the end of the first section of the tab. Fields that already exist keep their position, and ensureFieldPrepended() still puts them at the top.

While in there, ensuring a field that lives inside an imported fieldset now writes the config override back to the section the import is actually in. Previously the override could end up as a duplicate import descriptor in a different section whenever the import wasn't in the fallback section.

Before	After

Fixes #13644

🤖 Generated with Claude Code

[jasonvarga]

This is a questionable change.

I'd expect a programmatically-added thing to be appended to the end of the things. I'll let this through for now but part of me feels there might be some backlash. We'll see what feedback we get - there's a chance this'll need to be reverted.

[mynetx]

Thanks @jasonvarga. I agree with you, but if not let's do some brainstorm how to see alternative solutions for #13644. 🤔