Use bundled aggregations from server side search for threads

### Your use case

#### What would you like to do?

#4387 implemented handling search results for threads, but only from local information. [MSC3666](https://github.com/matrix-org/matrix-doc/pull/3666) expanded the set of endpoints which include bundled aggregations to also include the `/search` endpoint. It would be good to use information returned from the server for finding threads in case the threaded events are not known locally.

#### Why would you like to do it?

It is likely that the homeserver has a more complete view of the room than the client (i.e. it can see the entire room history, which might not have been pulled locally). This should help render search results in a more accurate manner.

#### How would you like to achieve it?

Using the bundled aggregations as described in MSC2675/MSC3440, as is done for other APIs.

### Have you considered any alternatives?

N/A

### Additional context

_No response_