EmbeddedEventLoop/Channel: check correct thread (#2951)

### Motivation:

`EmbeddedChannel` & `EmbeddedEventLoop` currently violate `Sendable`.
They should store the current thread in `init` and then implement
`inEventLoop` and other functions with a check that the current thread
is correct.
Since NIO 1.0 `Embedded*` were always documented to not be thread-safe
but we should finally police this.

### Modifications:

- Implement the thread check
- For now, just warn (soon hopefully crash)
- Delete EmbeddedScheduledCallbackTests (#2950)

### Result:

- Less Embedded* abuse
- fixes #2949