[Bug]: activity parsing fails and shows an error to the user

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I've searched it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

the activities shown in main dialog might be replaced by an error message about activity message failing to be parsed by the client

### Steps to reproduce

1. Enable activity app on your server
2. Sync
3. At a point the activity list may show errors

### Expected behavior

All activities sent by the server are decoded correctly

### Which files are affected by this bug

n/a

### Operating system

Linux

### Which version of the operating system you are running.

linux

### Package

Compiled it myself

### Nextcloud Server version

31.0.4

### Nextcloud Desktop Client version

master branch

### Is this bug present after an update or on a fresh install?

Fresh desktop client install

### Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

### Are you using an external user-backend?

- [ ] Default internal user-backend
- [ ] LDAP/ Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell

```

### Additional info

_No response_

No affected files, tests, entry points, logs, or failing activity payloads are identified. Start by reproducing the error with the activity app on Nextcloud Server 31.0.4 and the desktop client's master branch, then capture the activity data and client context. Done means identifying the parsing failure and confirming that server-sent activities display without an error.

Written by the indexing model from the issue text.