Collateral change: ScriptEventVehicleAutoReplaced is now only called when the head engine changes, so only when the VehicleID of the consist changes.

Motivation / Problem

#9256
While spawning an AI event, owner and Index of a vehicle are accessed after the vehicle has already been sold.

Description

The event is now spawned before the vehicle is sold. (but when success is already known)

This PR also changes whether the event is spawned when replacing wagons:

• The AIEvent contains the old and new vehicle Id of the consist head.
• When only auto-replacing wagons, the head does not change, so the event would say oldid==newid.
• No idea whether an event is intented/useful here.

Limitations

Original intent of event when replacing wagons is unknown.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker