"Invalid" `m.direct` global account data causes issues when handling notifications.

... I think? I saw a bunch of SSS requests, and looking at the EXA logs I found: `Failed to deserialize a global account data event: leading sigil is incorrect or missing at line 1 column 553 event_type="m.direct"`.

Indeed, in my `m.direct` I see an entry like `{"xxxx@jki.re": [...]}`, which is probably the thing that is causing issues.

I'm not sure exactly how much of a problem this (I have push disabled on EXA, so not even sure why its trying to handle notifications).

Start by tracing handling of the `m.direct` global account data event and the notification path described in the issue, using the reported deserialization error and EXA logs as reproduction clues. Determine how malformed account data should be handled and verify that notification processing no longer fails or produces unexpected SSS requests.

Written by the indexing model from the issue text.