fix: soft synchronization errors and improperly marking in-scene placed on spawned objects

[NoelStephensUnity]

Purpose of this PR

This PR resolves three regression bugs:

• The active scene was not being serialized as the 1st scene and could be loaded on the client-side after other scenes loaded from the received SceneEventData had been loaded resulting in those scenes being unloaded when client synchronization mode is set to LoadSceneMode.Single.
• Recent improvements to how we handle marking NetworkObjects as in-scene placed introduced an editor only issue where it could end up setting already dynamically spawned instances as in-scene placed.
• Specific edge case scenarios could result in no network prefab being found and upon logging a warning message during NetworkObject.Deserialization it could throw an exception when the searched for prefab's NetworkObject resulted in returning null.

Jira ticket

Jira pending...

Fix: #4049

Changelog

• Fixed: WIP

Documentation

• No documentation changes or additions were necessary.

Testing & QA (How your changes can be verified during release Playtest)

Functional Testing

Manual testing :

• Manual testing done
  • A manual test was added to the internal NGO Examples repository on the main branch. (pending push)

Automated tests:

• Covered by existing automated tests
• Covered by new automated tests
  • WIP: Several tests need to be written to cover these edge cases.

Does the change require QA team to:

• Review automated tests?
• Execute manual tests?
• Provide feedback about the PR?

If any boxes above are checked the QA team will be automatically added as a PR reviewer.

Backports

No back port is required.