WebR: a session that has no private ssk cached should not be able to accept a verification request

[BillCarsonFr]

... If it can, the verification dance will work but the session will not be cross-signed

See https://github.com/element-hq/element-web/issues/21919

[BillCarsonFr]

Other occurence https://github.com/element-hq/element-web/issues/27502

[kegsay]

https://github.com/element-hq/element-web/issues/27623 is similar, but this time it's the backup key that is missing, the other 3 are there.

[richvdh]

Related: https://github.com/element-hq/element-meta/issues/2642