Event is not failed when creating a spam checker module that filters federated rooms

This issue has been migrated from [#16727](https://github.com/matrix-org/synapse/issues/16727).

---

### Description

I am trying to create a module that blocks federation of rooms between two matrix servers.
Logics is based on m.room.member event type and determines source server and destination server.
If source server is different than destination server and room id is not in allowed rooms, then return forbidden.

Rooms are provided as config in values file:
```
- module: federation_event_checker.FilterFederatedRooms
config:
fedrooms_allowed:
- "!AanXREkoCFRofkefum:matrix2.dev.development.int" #matrix2tomatrix
```

Python module

```
async def check_event_for_spam(self, event: "synapse.events.EventBase"):

event_dict = event.get_dict()

try:
event_type = event_dict.get('type', None)
if event_type == 'm.room.member':
print("Identifier passed event type")
event_content = event_dict.get('content', None)
print("Identifier print event content",event_content)
if event_content["membership"] == 'invite':
print("Identifier determine source server and destination server")
source_server = event.origin
dest_server = event.state_key.split(":")[1]
fedroom = event_dict.get('room_id', None)
if source_server != dest_server:
if fedroom not in self.fedrooms_allowed:
print("Identifier: For room id", fedroom, " federation IS NOT allowed with other matrix servers")
return synapse.module_api.errors.Codes.FORBIDDEN
except Exception:
print("Identifier exception for event", event_dict)
return synapse.module_api.errors.Codes.FORBIDDEN
return synapse.module_api.NOT_SPAM
```

Logs from execution

```
2023-12-05 06:45:20,474 - synapse.util.metrics - 163 - DEBUG - PUT-62- Entering block federation_event_checker.FilterFederatedRooms.check_event_for_spam

2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier passed event type
2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier print event content {'displayname': 'xxxxx', 'membership': 'invite'}
2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier determine source server and destination server
2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier For room id !zznlJqNveedzoyXqee:matrix2.dev.development.int federation IS NOT allowed with other matrix servers
2023-12-05 06:45:20,475 - synapse.util.metrics - 176 - DEBUG - PUT-62- Exiting block federation_event_checker.FilterFederatedRooms.check_event_for_spam
2023-12-05 06:45:20,475 - synapse.federation.federation_base - 143 - WARNING - PUT-62- Event contains spam, soft-failing $rQtlVB1HGJsli1eRemKWawHYO4hL0L9RGCA5zPbXISE
```

Last line is very important: "**Event contains spam, soft-failing $rQtlVB1HGJsli1eRemKWawHYO4hL0L9RGCA5zPbXISE**"
It says that the event contains spam and soft failing but still invite can be sent to the other user and the other use may accept the invite then talk.

### Steps to reproduce

- Having two federated matrix servers: matrix and matrix2
- Creating a new room on matrix2 server (room is not in allowed list)
- Inviting a user to the room from matrix server
- User from matrix server is able to accept the request and join the room but it should not be able

### Homeserver

local homeserver

### Synapse Version

{"server":{"name":"Synapse","version":"1.92.3"}}

### Installation Method

Docker (matrixdotorg/synapse)

### Database

Postgresql

### Workers

Single process

### Platform

Rancher - Kubernetes

### Configuration

- module: federation_event_checker.FilterFederatedRooms
config:
fedrooms_allowed:
- "!AanXREkoCFRofkefum:matrix2.dev.development.int" #matrix2tomatrix

### Relevant log output

```shell
2023-12-05 06:45:20,474 - synapse.util.metrics - 163 - DEBUG - PUT-62- Entering block federation_event_checker.FilterFederatedRooms.check_event_for_spam

2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier passed event type
2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier print event content {'displayname': 'vlad_ad', 'membership': 'invite'}
2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier determine source server and destination server
2023-12-05 06:45:20,475 - twisted - 275 - INFO - PUT-62- Identifier For room id !zznlJqNveedzoyXqee:matrix2.dev.development.int federation IS NOT allowed with other matrix servers
2023-12-05 06:45:20,475 - synapse.util.metrics - 176 - DEBUG - PUT-62- Exiting block federation_event_checker.FilterFederatedRooms.check_event_for_spam
2023-12-05 06:45:20,475 - synapse.federation.federation_base - 143 - WARNING - PUT-62- Event contains spam, soft-failing $rQtlVB1HGJsli1eRemKWawHYO4hL0L9RGCA5zPbXISE
```

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

_No response_