Improper avatar thumbnail size is returned by Synapse when using Element Desktop on some operating systems

### Description

This issue was originally filed in the Element Desktop repository, but it was determined to likely be an issue with Synapse. Relevant information can be found here: https://github.com/element-hq/element-desktop/issues/2828

### Steps to reproduce

Steps listed here https://github.com/element-hq/element-desktop/issues/2828

### Homeserver

another homeserver

### Synapse Version

v1.146.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

PostgreSQL

### Workers

Multiple workers

### Platform

Dedicated server, Ubuntu 24.04

### Configuration

`enable_media_repo: true`
(in addition to media repository worker)

### Relevant log output

```shell
Observed no relevant logs despite setting to DEBUG level
```

### Anything else that would be useful to know?

_No response_

Start with the reproduction steps and evidence in element-hq/element-desktop#2828, then trace Synapse's media repository and avatar-thumbnail handling with enable_media_repo and the reported worker setup. Reproduce on Synapse v1.146.0 using Docker and PostgreSQL, and confirm that the returned thumbnail dimensions match the expected Element Desktop size.

Written by the indexing model from the issue text.