Carla & CarlaTools plugins fail to compile under bUseUnityBuild=false (latent IWYU bugs)

[youtalk]

Setup

• CARLA version: ue5-dev @ 1aae09e (post-fix(streaming): make ServerSession::CloseNow idempotent on ue5-dev #9761)
• Platform: Linux x86_64
• Unreal Engine: 5.5 (CARLA fork)
• Build config: bUseUnityBuild=false in ~/.config/Unreal Engine/UnrealBuildTool/BuildConfiguration.xml
• Targets affected: carla-unreal-editor (editor); the same files compile under bUseUnityBuild=true only because their unity-blob siblings transitively supply the missing headers.

Describe the bug
Four .cpp/.h files in the Carla and CarlaTools plugins rely on transitive includes pulled in by neighboring .cpp files in their unity blob. With bUseUnityBuild=false they fail to compile. Beyond the obvious "I can't build without unity" hit, this also penalizes ccache even under unity ON: small edits churn the unity blob composition and invalidate sibling translation units that don't truly depend on the edit.

Concrete errors (from a clean build with unity off):

Plugins/Carla/Source/Carla/BlueprintLibary/PostProcessJsonUtils.cpp:7
  #pragma once  -- stray, in a .cpp
  + use of FJsonObjectConverter / USceneCaptureComponent2D / FFileHelper
    / FPaths / IFileManager without the corresponding includes

Plugins/Carla/Source/Carla/BlueprintLibary/MeshToLandscape.cpp:345
  error: no matching function for call to 'LockRead'
  note: cannot convert argument of incomplete type 'FPhysScene *'
        (aka 'FPhysScene_Chaos *') to 'FChaosScene *'

Plugins/CarlaTools/Source/CarlaTools/Private/BlueprintLibrary/MeshToSplineActor.cpp:347
  error: no matching function for call to 'DestructItems'
  note: invalid application of 'sizeof' to an incomplete type 'FProcMeshTangent'

The bundled StreetMap plugin (carla-simulator/StreetMap, branch ue5-dev-carla) has the same class of bug in two more files (StreetMapImportBlueprintLibrary.cpp, StreetMapComponentDetails.cpp) and will be addressed by a companion PR in that repository.

Steps to reproduce

1. Set bUseUnityBuild=false in ~/.config/Unreal Engine/UnrealBuildTool/BuildConfiguration.xml.
2. export CARLA_CCACHE=1 && cmake --preset Development && cmake --build Build/Development --target carla-unreal-editor
3. Build fails on the files above.

Expected behavior
carla-unreal and carla-unreal-editor build green with bUseUnityBuild=false, so devs can iterate per-file and ccache can key on stable individual TUs.

Additional context
Fix only adds includes (and removes a stray #pragma once from a .cpp); no API or behavior change. PR to follow.