Introduce a new StreamingEditFileTool (#29733)

This pull request introduces a new tool for streaming edits. The
short-term goal is for this tool to replace the existing `EditFileTool`,
but we want to get this out the door as soon as possible so that we can
start testing it.

`StreamingEditFileTool` is mutually exclusive with `EditFileTool`. It
will be enabled by default for anyone who has the `agent-stream-edits`
feature flag, as well as people that set `assistant.stream_edits` to
`true` in their settings.

### Implementation

Streaming is achieved by requesting a completion while the `edit_file`
tool gets called. We invoke the model by taking the existing
conversation with the agent and appending a prompt specifically tailored
for editing. In that prompt, we ask the model to produce a stream of
`<old_text>`/`<new_text>` tags. As the model streams text in, we
incrementally parse it and start editing as soon as we can.

### Evals

Note that, as part of this pull request, I also defined some new evals
that I used to drive the behavior of the recursive LLM call. To run
them, use this command:

```bash
cargo test --package=assistant_tools --features eval -- eval_extract_handle_command_output
```

Or comment out the `#[cfg_attr(not(feature = "eval"), ignore)]` macro.

I recommend running them one at a time, because right now we don't
really have a way of orchestrating of all these evals. I think we should
invest into that effort once the new agent panel goes live.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>

crates/gpui/src/platform.rs

@@ -78,7 +78,7 @@ pub(crate) use test::*;
 pub(crate) use windows::*;
 
 #[cfg(any(test, feature = "test-support"))]
-pub use test::TestScreenCaptureSource;
+pub use test::{TestDispatcher, TestScreenCaptureSource};
 
 /// Returns a background executor for the current platform.
 pub fn background_executor() -> BackgroundExecutor {