Add CallbackHelper lifecycle handling and fix unit test execution

[OswaldoApexSystems]

EOSU-1013

-Introduced a new CallbackHelper class to centralize notification callback lifecycle management and prevent stale callback collisions when the EOS Platform is recreated.

-Updated EOSManager and FriendTest to correctly integrate with the new callback lifecycle behavior, ensuring callbacks are cleared at the proper time during test teardown and platform shutdown.

-These changes fix failures encountered when running the full unit test suite by ensuring isolation between tests and preventing leftover EOS notification state from leaking across test runs.

[matt-clarke]

This docs file covers a couple of points which are a bit of a concern:

• "It is not the default because if it were, then any edits to the EOSConfig file would require a reboot of the Unity Editor"
  • Can you check if this is accurate?
  • If it is then we would need to think of a solution to this, perhaps when you save the EOSConfig it prompts you to restart the Editor (after checking the new define mentioned in the comment below)
  • I suspect we also need a Unity hook to check if the file was modified outside of Unity (user could've edited it by hand, or pulled latest changes from their repo)
• "In the past, not unloading the EOS SDK could also cause issues with state from the previous Unity run not being properly cleaned up."
  • This is the opposite of what we're seeing right? I'm wary that there's some edge case issues that we haven't seen with this initial testing. It could definitely be a misunderstanding though
  • The commit message where it was added also says the same, so it doesn't appear to just be a mistake in the docs (It also carries with it the potential for "leaked" EOS State to carry over between play in editor runs.)

[matt-clarke]

With this change to always define EOS_DO_NOT_UNLOAD_SDK_ON_SHUTDOWN in Editor this means that this docs file is out of date.

The change above also removes the ability for developers to control whether it's enabled in Editor or not for Windows.

One option would be to change the above code to this, and update the docs to refer to this define instead:

#if UNITY_EDITOR && (!UNITY_EDITOR_WIN || !EOS_UNLOAD_SDK_ON_SHUTDOWN)
#define EOS_DO_NOT_UNLOAD_SDK_ON_SHUTDOWN
#endif

[matt-clarke]

Honestly it would be nice to update these defines whilst we're here:

• EOS_DO_NOT_UNLOAD_SDK_ON_SHUTDOWN_IN_EDITOR
• EOS_UNLOAD_SDK_ON_SHUTDOWN_IN_WIN_EDITOR

[matt-clarke]

Keeping this comment active because this docs file needs updating with the new define