timeline: Warn when constructing thread timeline without threading support in client

~~The [log](https://github.com/user-attachments/files/22891540/iamb.log) with level trace.~~

~~I am using the latest commit (`588d60465`) of `matrix-sdk-ui` in a modified version of [iamb](https://github.com/ulyssa/iamb).~~

~~The only sync running is `SyncService` without any subscriptions by room id.~~

~~I have a room open in two timelines, one with main focus and one with thread focus.~~

- ~~At `2025-10-13T18:35:12.561343Z` I send a new message to the thread timeline. (ignore that is says `editing`)~~
- ~~A local echo is inserted at some point in time.~~
- ~~At `2025-10-13T18:35:12.845689Z` the server returns the new event id.~~
- ~~At `2025-10-13T18:35:12.847761Z` the message isn't inserted in the live timeline because it is in a thread.~~

~~It seems something is handled incorrectly because the local echo in the thread timeline is not replaced and the thread summary aggregation in the main timeline is not updated.~~

~~possibly related to #5773~~

When creating `Timeline` with thread focus from a client without [`ClientBuilder::with_threading_support`](https://matrix-org.github.io/matrix-rust-sdk/matrix_sdk/struct.ClientBuilder.html#method.with_threading_support) there should be an indication (error or log entry) that new messages wont be handled correctly.

Start by reading ClientBuilder::with_threading_support and the Timeline construction path for thread focus. Check how threading support is represented and how existing errors or logs are emitted; done means a client without that support receives a clear indication before new thread messages are mishandled.

Written by the indexing model from the issue text.