Fix typed-data contract violation in AddressGsearchItem
The custom getValue(): string override on AddressGsearchItem broke the
TypedDataInterface contract, which requires getValue() to return the
array of all field properties. This caused field_inheritance (and any
other consumer relying on the standard typed-data API) to receive a
flat string instead of the property array, leading to inherited items
being created with all subfields NULL and rendering as empty.
Replace the override with an override of getString() instead, which is
the conventional typed-data method for returning a string
representation of a field item. The default Map::getString() joins all
properties with commas (uuid, coords, etc.), so we override it to
return just the main 'value' property — the human-readable address.
Drop getValue() from AddressGsearchItemInterface; callers should use
$item->getString() (or $item->value / $item->get('value')->getValue()
for the raw main property) instead.
BREAKING CHANGE: Callers using $item->getValue() to get the address
string must switch to $item->getString(). The internal formatter and
tests are updated accordingly.