legacy `de.sorunome.msc2409.push_ephemeral` flag no longer honored in appservice handler

[weebl2000]

Description

Had an appservice configured to use de.sorunome.msc2409.push_ephemeral: true - after upgrading from v1.155.0 to v1.156.0 the syncing of keys/messages died.

Steps to reproduce

• have an appservice and use config de.sorunome.msc2409.push_ephemeral: true
• verify it works on v1.155.0
• verify it breaks with v1.156.0

Homeserver

my own

Synapse Version

v1.156.0

Installation Method

Debian packages from packages.matrix.org

Database

psql 18

Workers

Single process

Platform

u24.04 amd64

Configuration

No response

Relevant log output

-

Anything else that would be useful to know?

No response