Enable key upload to backups where we have the decryption key

[ajbura]

Fixes #4676

Signed-off-by: Ajay Bura [email protected]

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[richvdh]

Putting this on hold while we try to understand what bug it is trying to fix, per #4676 (comment).

In any case, we'll need to see an integration test which demonstrates the problem.

[richvdh]

ok, the fix looks like it makes sense, but the main thing this is missing is a regression test.

Please could you take a look at matrix-js-sdk/spec/integ/crypto/megolm-backup.spec.ts. In there there are some existing tests for checkKeyBackupAndEnable: please could you take a look at them and see if you can add another which covers your usecase.