Thread notifications don't use the parent room's conversation channel / custom sound

### Steps to reproduce

1. Open a room that notifies for all messages.
2. Send a message in that room so the room's conversation shortcut is created.
3. In Android settings, go to Notifications > Conversations, find the room's conversation, and set a custom sound.
4. Have someone reply in a thread in that room.
5. Observe the incoming thread notification.

### Outcome

#### What did you expect?

The thread notification should play the custom sound configured for that room's conversation, matching how regular (non-thread) messages behave.

#### What happened instead?

The thread notification plays the default sound and appears under the default "Noisy notifications" channel instead of the room's conversation channel. There is no way for the user to set a per-room sound for thread notifications.

### Your phone model

Google Pixel 8 Pro

### Operating system version

Android 17 (build 2026080501)

### Application version and app store

Element X 26.08.0 (GitHub release)

### Homeserver

Synapse (self-hosted)

### Will you send logs?

No

### Will you submit a Pull Request?

Yes

Start by tracing how thread notifications are created and compare that path with regular room-message notifications, focusing on Android conversation-channel and custom-sound selection. Reproduce the issue using the listed steps, then verify that a thread notification appears in the parent room's conversation channel and uses its configured sound.

Written by the indexing model from the issue text.