fix(streaming): make ServerSession::CloseNow idempotent on ue5-dev

[SunayKulkarni]

Description

Port of #9740 (ue4-dev) to ue5-dev.

Adds std::atomic_bool _is_closed{false} to ServerSession and guards
CloseNow() with _is_closed.exchange(true) so the close path runs exactly
once, preventing a double-disconnect from evicting a still-alive subscriber
from the world-snapshot stream.

Fixes #9759

Where has this been tested?

• Platform(s): Not compiled/tested locally — this is a straight port of the reviewed and approved fix from fix(streaming): make ServerSession::CloseNow idempotent (one client drop no longer stalls others) #9740
• Python version(s): N/A
• Unreal Engine version(s): UE5 (ue5-dev)

Possible Drawbacks

None expected. The change is minimal (~3 lines), platform-independent LibCarla C++, and matches the fix already approved for ue4-dev.

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This change is 

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update our CHANGELOG.md based on your changes.

[LuisPovedaCano]

This port looks good to me.
Approving to enqueue in CI, but please fix the comment.

[LuisPovedaCano]

Could you please add and space between the #include and atomic to match the code style fo the rest of the file?

[SunayKulkarni]

sure thing