bug: non-admin editing a public model silently makes it private

An admin creates a workspace model and shares it publicly. A user with write access to that model, but without the "Models Public Sharing" permission, opens the editor, changes something unrelated like the name or the base model, and saves. The model is now private. The public share is gone and nobody is told.

Steps:

1. As admin, create a workspace model, set it to Public, give a normal user write access.
2. Leave the user permission for public model sharing off (the default).
3. As that user, open the model in the workspace editor, change name and base model, save.
4. As admin, open the model again: it is Private.

Expected: the edit is saved and the model stays public. The user could not have added the public share themselves, so a save that does not touch sharing must not remove it either.

Actual: the update endpoint runs every access grant the editor sends back through the same permission filter that applies when a user adds grants. The editor sends the existing grants back unchanged on every save, so the public grant that the admin set gets stripped and the model is written back as private.

Reproduced on current dev