Fix citizen follow through vehicle rides

[mayor-modder]

When a followed citizen boards a vehicle, follow mode could accidentally start treating the vehicle as the thing being followed. This keeps the person as the real target while still letting the camera ride along with the vehicle until they get out.

Summary

• Keep the selected citizen as the logical follow target while resolving a separate camera attachment target for entered vehicles.
• Preserve follow state when a followed citizen boards and leaves a vehicle instead of switching permanently to the vehicle.
• Add focused tests for follow-target state transitions.

Validation

• dotnet test Tests/FollowTargetStateTests.csproj --no-restore (2 passed)
• dotnet build FirstPersonCameraContinued.csproj /p:ModPublisherCommand=Update /m:1 /nodeReuse:false /p:UseSharedCompilation=false (0 errors; existing nullable warnings remain)
• Local gameplay retesting followed a citizen through subway boarding and disembarking without losing the citizen follow target.

Notes

This is the base PR for a small stacked follow-up: mayor-modder#4.