Fix Prop Factory - Enable prop spawning from Python API

[LuisPovedaCano]

This PR is related with this content PR

Problem

The prop factory was not working - props could not be spawned from the Python API. Attempting to spawn any prop would fail silently or return errors.

Root Cause

Three main issues prevented props from spawning:

1. Missing Class Assignment: MakePropDefinition() in ActorBlueprintFunctionLibrary.cpp was not assigning Definition.Class, leaving it null. This caused SpawnActor() to fail the validity check.

2. No Mesh Path Propagation: There was no mechanism to pass the mesh path from the prop definition to the spawner, making it impossible to load the correct mesh at spawn time.

3. Incomplete SpawnActor Implementation: PropActorFactory::SpawnActor() returned UnknownError immediately without any actual spawning logic.

Testing

Tested in Ubuntu 22 in Town10.

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