fix: NetworksSenemanager order of operations and scene migration preprocess

[NoelStephensUnity]

This resolves two issues:
Moving the client-side logic to first mirror the server's current active scene prior to spawning and synchronizing NetworkObjects during the initial client synchronization.

Pre-processing NetworkObject scene migration table to assure all NetworkObjects are still spawned, removing those that are no longer spawned (i.e. despawned in the same frame), and if no NetworkObjects remain for each scene handle entry not sending a scene migration notification.

Changelog

• Changed: connecting clients being synchronized now switch to the server's active scene before spawning and synchronizing NetworkObjects.
• Fixed: issue where a server would include scene migrated and then de-spawned NetworkObjects to a client that was being synchronized.

Testing and Documentation

• Includes integration test update to NetworkObjectSceneMigrationTests.MigrateIntoNewSceneTest.
• No documentation changes or additions were necessary.