Element-R: Verifying against a device that does not have the private SSK claims to succeed, but does not

[BillCarsonFr]

STR:

• Sign in on a new device
• Verify against a device which, for some reason, does not have a copy of the private cross-signing keys. (It's not entirely clear how to get into this situation; one way to do it might be to verify against another device which doesn't correctly implement secret sharing).

Observe success message:

However, the new device is not actually cross-signed, and doesn't receive the secrets.

• At the very least, the session that has no private SSK should not accept the verification request
• We should probably also not offer verifying against a device as a potential verification method if the other devices lack private SSKs; however this may be tricky.
• We should probably not report verification success until the cross-signature and cross-signing keys are received.

See #21919

[BillCarsonFr]

Other occurence #27502

[kegsay]

#27623 is similar, but this time it's the backup key that is missing, the other 3 are there.