[jasonvarga]

This PR makes Tab::expandedSections() public so code other than the CP publish form can use fieldset section expansion (added in #14297). It's a refactor; publish output is unchanged.

Previously the expansion lived in private methods that returned publish arrays, so only Tab::toPublishArray() could use it. Everything else calls $tab->sections(), which doesn't split sections around sectioned fieldset imports.

Changes

• Tab::expandedSections() is now public and returns a Collection of Section objects. Tab::toPublishArray() maps them to publish arrays.
• Section accepts an optional second constructor argument of already-resolved Fields, which fields() returns when set. Imported fieldset sections use this, so their fields keep the import's prefixes and config overrides.
• When a Section has resolved fields, toPublishArray() maps them directly instead of going through Fields::toPublishArray(). That method's Blink key ignores field prefixes, so fields that differ only by prefix could otherwise share cached output. The old code avoided it the same way.
• Tab::sections() and Tab::fields() are unchanged.

Backwards compatibility

• I compared Tab::toPublishArray() output before and after with one large fixture, and it's identical. The fixture covered nested imports with inner and outer prefixes, config overrides on prefixed imports, collapsible/collapsed sections, empty sections, section_behavior: flatten, and multiple imports of the same fieldset.
• The existing expansion tests pass unmodified.
• expandedSections() going from private to public would only clash with an addon subclass of Tab that declares its own expandedSections().
• If a fieldset section has a non-boolean collapsible/collapsed value (e.g. a quoted 'yes'), it now ends up as a boolean in the publish array, the same as blueprint sections already do. YAML booleans are unaffected.

Why

The forms-2 work (the forms API and form:create) will use expandedSections() so forms show fieldset sections the same way the CP does.

Possible follow-ups

These still flatten sectioned imports via $tab->sections() on 6.x. Switching them to expandedSections() would be a behaviour change, so they're left alone here:

• Forms\Tags (form:create sections)
• Auth\UserTags (user register/profile forms)