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/assistant_tools/src/edit_file_tool.rs

@@ -282,7 +282,7 @@ pub struct EditFileToolCard {
 }
 
 impl EditFileToolCard {
-    fn new(path: PathBuf, project: Entity<Project>, window: &mut Window, cx: &mut App) -> Self {
+    pub fn new(path: PathBuf, project: Entity<Project>, window: &mut Window, cx: &mut App) -> Self {
         let multibuffer = cx.new(|_| MultiBuffer::without_headers(Capability::ReadOnly));
         let editor = cx.new(|cx| {
             let mut editor = Editor::new(
@@ -323,7 +323,7 @@ impl EditFileToolCard {
         }
     }
 
-    fn set_diff(
+    pub fn set_diff(
         &mut self,
         path: Arc<Path>,
         old_text: String,
@@ -343,6 +343,7 @@ impl EditFileToolCard {
                         .hunks_intersecting_range(Anchor::MIN..Anchor::MAX, &snapshot, cx)
                         .map(|diff_hunk| diff_hunk.buffer_range.to_point(&snapshot))
                         .collect::<Vec<_>>();
+                    multibuffer.clear(cx);
                     let (_, is_newly_added) = multibuffer.set_excerpts_for_path(
                         PathKey::for_buffer(&buffer, cx),
                         buffer,