Fix Allow any valid user access to public Dataset through SDK

[mjcurran]

SFDS-320
Fix permission issues trying to access assets of public datasets

• If a Dataset is public then allow a valid user to have Viewer permissions to download through SDK.

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Note

High Risk
Widens authorization: any logged-in user can view/download assets on public datasets and captures. Incomplete coverage of deprecated FK query paths could also leave access inconsistent.

Overview
Authenticated users now get viewer access to public datasets and captures (and their nested files/captures) without an explicit share, so SDK downloads of public dataset assets work.

UserSharePermission.get_user_permission_level falls back to PermissionLevel.VIEWER when the item is public and not deleted. Capture/file access helpers and the accessible-files/captures querysets treat is_public like ownership or a share on M2M relationships (deprecated FK paths are unchanged).

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[lucaspar]

Thanks. It'd be good to add some regression tests to make sure this keeps working in the future.