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

Medium Risk
Changes authorization rules for datasets, captures, and files; incorrect is_public handling could expose or over-restrict assets, though scope is limited to the intended public flag.

Overview
Fixes SDK and API access for public datasets, captures, and linked files when there is no explicit UserSharePermission row.

Permission model: UserSharePermission.get_user_permission_level now returns viewer for any authenticated check path when the target dataset or capture has is_public=True (and is not deleted), after owner and explicit share checks fail.

Access control utilities: user_has_access_to_capture, user_has_access_to_file, and the get_accessible_*_queryset helpers treat is_public like an implicit grant— including nested paths (files on public captures, files on captures in public datasets, files linked to public datasets, and directly public files).

Tests: Adds coverage that an unshared user sees public M2M-linked files and captures in the accessible querysets.

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[lucaspar]

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

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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