[6.x] Hide section behavior control where imported sections can't be preserved

[jaygeorge]

Fixes #15602

Imported fieldset sections are only preserved when the import sits directly in a blueprint (Tab::toPublishArray()). Anywhere else, inside Replicator/Bard sets, Grid/Group fields, or another fieldset, they're always flattened. The CP still offered the "Section Behavior" control in those places, so choosing "Preserve" looked like it should work but did nothing.

This PR:

• Hides the Section Behavior control when linking or configuring a fieldset import anywhere sections can't be preserved.
• Shows the "Ignoring Sections" badge on those imports, so it's clear the fieldset's sections will be flattened.
• Stops "Link Existing" from writing section_behavior in those contexts.

The contexts are detected with the existing isInsideSet and isInsideConfigFields injections, plus a new isInsideFieldset provided by the fieldset editor. The check lives in one PreservesImportedSections mixin.

This only changes the UI. Supporting collapsible sections inside sets would be a separate feature.