fix: Missing contexts

[bitsandfoxes]

We moved the collection of MainThreadData out of the MonoBehaviour's Awake call in #1802
I'm actually not 100% why that worked in the first place. The Awake happens past the SceneLoad so the contexts should not have been available at that time either.

Why is it broken

• The SDK initializes during RuntimeInitializeLoadType.SubsystemRegistration
  

  sentry-unity/package-dev/Runtime/SentryInitialization.cs

  Line 54 in 03af53d

  [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)]

• The SDK registers the integrations on the options
• The UnityScopeIntegration gets registered and populates the contexts
  

  sentry-unity/src/Sentry.Unity/Integrations/UnityScopeIntegration.cs

  Lines 46 to 58 in e7a45a9

  	PopulateSdk(scope.Sdk);
  	PopulateApp(scope.Contexts.App);
  	PopulateOperatingSystem(scope.Contexts.OperatingSystem);
  	PopulateDevice(scope.Contexts.Device);
  	PopulateGpu(scope.Contexts.Gpu);
  	var unity = new Protocol.Unity();
  	PopulateUnity(unity);
  	scope.Contexts.Add(Protocol.Unity.Type, unity);
  	PopulateTags(scope.SetTag);
  	PopulateUser(scope);
  	}

• The MainThreadData has not been collected at this point because this happens during BeforeSceneLoad
  

  sentry-unity/src/Sentry.Unity/MainThreadData.cs

  Lines 75 to 76 in f1fe17a

  	[RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
  	private static void CollectData() => CollectData(SentrySystemInfoAdapter.Instance);

• The contexts are missing

How to fix it

Instead of relying on the Unity RuntimeInitialize we can just collect the contexts right before initialization

sentry-unity/src/Sentry.Unity/SentryUnitySDK.cs

Line 31 in e7a45a9

MainThreadData.CollectData(SentrySystemInfoAdapter.Instance);

Everything is nice and explicit.
Updated our smoketest so contexts won't go missing anymore.