Search in E2EE rooms does not find words by part of a word in Element Web and Element Desktop

Nobody has claimed this yet.

Steps to reproduce:
1. Open Element Web or Element Desktop.
2. Open an end-to-end encrypted room.
3. Send a message containing the word "testing".
4. Send another message containing the word "test".
5. Open search in this room and type "test".
6. Do the same in a non-encrypted room and compare results.Outcome:

#### What did you expect?
Typing part of a word, for example "test", should find messages containing both "test" and "testing". This is how search works in non-encrypted rooms.

#### What happened instead?
In encrypted rooms, search only finds the exact word "test". The message with "testing" is not found. Users have to type the complete exact word to find anything. This makes search in E2EE rooms almost useless.

Additional context:
The Element Web issue tracker redirected me here. The problem is in Seshat which handles search for E2EE rooms in both Element Web and Element Desktop.

Start by reproducing the comparison between encrypted and non-encrypted room search described in the issue, then trace Seshat's encrypted-room search entry point. The change is complete when searching for "test" finds both the exact word and messages containing "testing", matching non-encrypted room behavior.

Written by the indexing model from the issue text.