fix: server not tracking scene handles for pre-loaded scenes

[NoelStephensUnity]

Up-port of #4145

Purpose of this PR

Scenes that were already loaded when a session starts were added to ScenesLoaded but never to the server-to-client scene handle tables, so UnloadScene passed its ScenesLoaded check and then failed inside RemoveServerClientSceneHandle. The scene unloaded on every peer, but an error was logged and the ScenesLoaded entry was never removed.

The client-server branch of the NetworkSceneManager constructor now registers those scenes the same way InitializeScenesLoaded does for distributed authority.

NetcodeIntegrationTestHelpers registered the test runner scene in ServerSceneHandleToClientSceneHandle unconditionally, which now collides with the constructor already having added it, so that add is guarded.

fix: #4140

Jira ticket

(Needs ticket)

Changelog

• Fixed: Issue where scenes additively loaded before a session started were tracked as loaded on the server but had no scene handle entries, which caused NetworkSceneManager.UnloadScene to log an error and leave the scene registered as loaded even though it unloaded on all peers.

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

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?
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Up-port

No up-port required.

Backports

No backport required.

[u-pr]

💡 Harness Review

The change consistently registers pre-start server scenes through the existing bidirectional handle helper while preserving loaded-scene population, and the accompanying test/helper adjustment matches that lifecycle. I examined the constructor, handle update/removal contract, scene population paths, and the startup test coverage.

_{Reviewed commit dab835f}

_{🤖 Helpful? 👍/👎}

[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@              Coverage Diff               @@
##           develop-3.x.x    #4146   +/-   ##
==============================================
  Coverage          78.01%   78.01%           
==============================================
  Files                153      153           
  Lines              26260    26260           
==============================================
  Hits               20486    20486           
  Misses              5774     5774           

Flag	Coverage Δ
NGOv2_project_testproject_ubuntu_pinnedTrunk	77.70% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...cts/Runtime/SceneManagement/NetworkSceneManager.cs	79.49% <100.00%> (ø)

Components	Coverage Δ
com.unity.netcode.gameobjects	78.01% <100.00%> (ø)

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