Gov. Maura Healey’s administration expects to spend more money than originally anticipated over the next fiscal year on the emergency shelter system housing migrants and local families, according to revised estimates released this month. Shelter-related costs are now projected to top $1 billion in fiscal year 2025 if caseloads remain the same, an increase from the $915 million state budget writers first said they expected to spend and a sign that officials do not forecast a slowdown in demand.