…6802)

## Problem

Profiling showed that during agent streaming, `thread.rs:1393:23` was
appearing constantly as a hotspot. The issue was that every single token
from the model triggered:
1. `this.update(cx, ...)` 
2. `handle_text_event()` (or thinking/redacted_thinking)
3. `cx.notify()`
4. UI re-render

This created significant foreground thread pressure, contributing to
~500ms delays visible in the profiler.

## Solution

Batch all immediately-available events using `now_or_never()` and
process them in a single `update()` call with one `notify()` at the end.

This approach is deterministic - it processes exactly what's available
right now, adapting naturally to network speed:
- When tokens arrive slowly, you get one at a time
- When they arrive in bursts, they batch automatically

## Changes

- Remove `cx.notify()` from `handle_text_event`,
`handle_thinking_event`, and `handle_redacted_thinking_event`
- Batch events in the streaming loop using `now_or_never()`
- Call `cx.notify()` once per batch instead of per-event
- Keep `cx.notify()` in `handle_tool_use_event` for immediate tool
feedback

Release Notes:
- Improve streaming tool call performance by batching UI updates.

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

```rust
// 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 `Option`s 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 ...