Fix getVehicleDummyPosition returning 0,0,0 for the second exhaust

[TheCrazy17]

Summary

Most vehicle models don't have an exhaust_second dummy, so the game places the second exhaust by mirroring the primary exhaust on the X axis. However, getVehicleDummyPosition(veh, "exhaust_second") returned the raw stored value (0, 0, 0 - the vehicle centre) instead of the mirrored position where the effects actually appear, e.g. after using resetVehicleDummyPositions.

The getter now applies the same mirroring fallback the game uses, so the reported position always matches the effective one. No visual or behavioural changes - only the returned value is fixed.

Motivation

Fixes #4761

Test plan

Using the script from the issue:

local veh = getPedOccupiedVehicle(localPlayer)
setVehicleDummyPosition(veh, "exhaust_second", 0, 0, -5000)
resetVehicleDummyPositions(veh)
print(getVehicleDummyPosition(veh, "exhaust_second"))
-- before: 0, 0, 0 | after: mirrored exhaust position (e.g. -0.39, -1.93, 0.15)

• Returns the mirrored primary exhaust position after a reset, matching where the exhaust/nitro effects render.
• Also works on freshly spawned vehicles without any set/reset.
• Vehicles whose model does define exhaust_second still return their real dummy position.
• Setting/resetting dummy positions and the effect rendering itself are unchanged.

Checklist

• Your code should follow the coding guidelines.
• Smaller pull requests are easier to review. If your pull request is beefy, your pull request should be reviewable commit-by-commit.