Space children cannot be assigned via `m.space.parent` event alone

### Steps to reproduce

1. Create a space (Space A)
2. Create another space (Space B)
3. Use /devtools to enable Developer Mode
4. Use "See room timeline" to access the Space B's timeline
5. Use /devtools in Space B's timeline to send a custom room state event:
- Event Type: `m.space.parent`
- State Key: `!:homeserver.tld`
- Content: `{"via":["homeserver.tld"],"canonical":true}`
6. Observe that Space B is not considered a sub-space of Space A

### Outcome

#### What did you expect?
Space B becomes a sub-space of Space A, as if it were associated using the `m.space.child` state in Space A, or using the "Add Space" UI to create it.

#### What happened instead?
Space B is not considered a sub-space of Space A, and does not appear as such in the Spaces side-bar

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

No