Bug 2056116 - Validate guids in add_many_with_meta

[jo]

add_with_meta / add_many_with_meta now validate the guid and reject it, or regenerate it under the new fixup_invalid_guids feature.

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[bendk]

Seems good to me, how confident are you that this fill fix the issue?

[jo]

This is not the fix for 2056116, but a guard against it.

It will ensure no sync-invalid id will be imported via the JSON -> Rust migrator on Desktop. Even though I am not hundred percent sure that this was the source of the problematic id, I think this is a useful check in any case.