Closes #ISSUE

Reproduction steps:

• Zed main branch
• Use Ollama (likely provider agnostic) in the agent panel with the following prompt: "Explore the codebase with subagents"
• Panic

thread 'main' (36268) panicked at C:\Users\username\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\futures-util-0.3.31\src\stream\unfold.rs:108:21:
Unfold must not be polled after it returned `Poll::Ready(None)`

Following the stack trace we get to Thread::run_turn_internal. I believe the panic happens in the following code which was introduced in #46802

// Collect all immediately available events to process as a batch
let mut batch = vec![first_event];
while let Some(event) = events.next().now_or_never().flatten() {
    batch.push(event);
}

Both Options get flattened, however the inner Option represents the end of the stream, after which polling the stream using .next() will result in a panic.

We could fix the logic in this particular spot, but I believe the simpler solution is to .fuse() the stream, which stops the stream from panic'ing even after it has ended. This also prevents misuse in the future.

The panic was introduces on main and did not land on a release yet, so no release notes.

Release Notes:

• N/A or Added/Fixed/Improved ...