Implement cloud fleet in-place update for provisioning fields

[r4victor]

Closes #3769

• Allow in-place update when resources, backends, regions, availability_zones, instance_types, spot_policy, or
  max_price change (previously forced fleet recreation)
• Fleet consolidation now detects existing instances that no longer match the updated spec and terminates
  idle/provisioning ones; busy instances are left alone until their job finishes
• Replacement instances are created in the same consolidation pass if the fleet drops below nodes.min
• Extract shared instance_matches_constraints() helper to avoid duplicating instance-matching logic between
  filter_instances() and fleet consolidation
• Fix _is_fleet_ready_for_consolidation fallback to use created_at instead of last_processed_at (could lead to consolidation not happening).