fix: only send scene migrations to observers, and spawn a NetworkObject shown after a migration in its authority's scene

[NoelStephensUnity]

Purpose of this PR

This PR fixes two scene migration issues for NetworkObjects that are hidden from some clients: those clients no longer log an error when the NetworkObject moves to another scene, and they spawn it in the authority's scene when it is shown to them later. Both fixes apply in client-server and distributed authority modes.

PR Scope:

• A scene migration (SceneEventType.ObjectSceneChanged) was sent to every connected client, including clients that did not observe the migrated NetworkObject and logged "Trying to synchronize NetworkObjectId but it was not spawned". It is now only sent to the clients that observe at least one migrated NetworkObject, and each client is only sent the NetworkObjects it observes. In distributed authority mode the owner applies the same filter; the CMB service is still sent every migrated NetworkObject, since it keeps the session state, and a migration the DAHost forwards is not filtered again.
• A NetworkObject shown with NetworkShow after it migrated into another scene, while hidden from that client, was instantiated in the client's active scene, since the client was never sent that migration. The client now moves the spawned NetworkObject into its server-side scene (its owner's scene in distributed authority mode), as it already does during its initial synchronization (NetworkSceneManager.SynchronizeNetworkObjectScene).

New NetworkObjectSceneMigrationObserverTests (4 tests, Host / DAHost / Server) cover both fixes, and a late joining client's synchronization of a NetworkObject that is not in the active scene.

Out of this PR's scope:

• None.

Jira ticket

TBD

Changelog

• Fixed: Issue where moving a NetworkObject into another scene made the clients that did not observe it log "Trying to synchronize NetworkObjectId but it was not spawned". The scene migration is now only sent to the clients that observe the NetworkObject.
• Fixed: Issue where a NetworkObject that was moved into another scene while hidden from a client spawned in that client's active scene when it was shown with NetworkShow, instead of the scene it is in on the server.

Documentation

• No documentation changes or additions were necessary.

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

Headless PlayMode runs on 6000.5.3f1.

Run	Passed	Failed
All of Unity.Netcode.RuntimeTests	4710	0
NetworkObjectSceneMigrationObserverTests	12	0

Functional Testing

Manual testing :

• Manual testing done

Automated tests:

• Covered by existing automated tests
• Covered by new automated tests

Does the change require QA team to:

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

Up-port

These fixes are part of #4176 on develop-3.x.x.

Backports

Not needed.

🤖 Generated with Claude Code

[u-pr]

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[u-pr]

💡 Harness Review

The change filters scene migrations by observer and moves newly shown objects into their authority’s scene, but the filtering leaves a reachable distributed-authority migration unsent to the session-state service.

_{Reviewed commit b69f17b}

_{🤖 Helpful? 👍/👎}

[NoelStephensUnity]

@u-pr please review again, your comment has been addressed.

[u-pr]

Thanks for the clarification and the updated comment in commit c9ba676.

The explanation holds: because a non-session-owner DA client does not inform the CMB service about objects only it observes (SpawnWithObservers = false), there is no stale state on the service to update at the time of migration. When NetworkShow is subsequently called, the resulting CreateObjectMessage transmits the current scene handle, and the newly added SynchronizeNetworkObjectScene call ensures the object is moved into the correct scene upon spawning.

The updated comment in NetworkSceneManager.cs accurately reflects that only the session owner needs to send migrations directly to the CMB service when peer observer lists are empty. The test coverage in NetworkObjectSceneMigrationObserverTests across Host, DAHost, and Server verifies these paths well.

Everything looks good.

_{🤖 Helpful? 👍/👎}

[codecov-github-com]

Codecov Report

Attention: Patch coverage is 90.54054% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...cts/Runtime/SceneManagement/NetworkSceneManager.cs	88.88%	5 Missing ⚠️
...eobjects/Runtime/SceneManagement/SceneEventData.cs	90.47%	2 Missing ⚠️

@@                Coverage Diff                @@
##           develop-2.0.0    #4185      +/-   ##
=================================================
+ Coverage          74.16%   74.21%   +0.04%     
=================================================
  Files                172      172              
  Lines              28144    28200      +56     
=================================================
+ Hits               20874    20929      +55     
- Misses              7270     7271       +1     

Flag	Coverage Δ
NGOv2_project_testproject_ubuntu	73.83% <90.54%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../Runtime/Messaging/Messages/CreateObjectMessage.cs	87.19% <100.00%> (+0.65%)	⬆️
...eobjects/Runtime/SceneManagement/SceneEventData.cs	77.73% <90.47%> (+0.51%)	⬆️
...cts/Runtime/SceneManagement/NetworkSceneManager.cs	79.83% <88.88%> (+0.33%)	⬆️

Components	Coverage Δ
com.unity.netcode.gameobjects	74.21% <90.54%> (+0.04%)	⬆️

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