Searching for word parts does not work

(copied from https://github.com/element-hq/element-web/issues/34940)

### Steps to reproduce

1. Have a chat with long words, e.g. "Spülmaschine"
2. Type part of the long word in the search field, e.g. "maschine"

### Outcome

#### What did you expect?
It should find chats containing "Spülmaschine"
#### What happened instead?
It finds nothing

### Operating system

Ubuntu

### Application version

Element: v1.11.80 Crypto-Version: Rust SDK 0.7.2 (7a21514), Vodozemac 0.7.0

### How did you install the app?

debian package

### Homeserver

matrix.org

### Will you send logs?

No

No file or test is named. Start by reproducing the search with a long word such as “Spülmaschine” and the fragment “maschine,” then trace the Rust search entry point and existing search tests. Done means searches for word parts return messages containing the full word, with a regression test covering the example.

Written by the indexing model from the issue text.