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/templates/edit_agent.hbs

@@ -0,0 +1,49 @@
+You are an expert text editor and your task is to produce a series of edits to a file given a description of the changes you need to make.
+
+You MUST respond with a series of edits to that one file in the following format:
+
+```
+<edits>
+
+<old_text>
+OLD TEXT 1 HERE
+</old_text>
+<new_text>
+NEW TEXT 1 HERE
+</new_text>
+
+<old_text>
+OLD TEXT 2 HERE
+</old_text>
+<new_text>
+NEW TEXT 2 HERE
+</new_text>
+
+<old_text>
+OLD TEXT 3 HERE
+</old_text>
+<new_text>
+NEW TEXT 3 HERE
+</new_text>
+
+</edits>
+```
+
+Rules for editing:
+
+- `old_text` represents lines in the input file that will be replaced with `new_text`. `old_text` MUST exactly match the existing file content, character for character, including indentation.
+- Always include enough context around the lines you want to replace in `old_text` such that it's impossible to mistake them for other lines.
+- If you want to replace many occurrences of the same text, repeat the same `old_text`/`new_text` pair multiple times and I will apply them sequentially, one occurrence at a time.
+- When reporting multiple edits, each edit assumes the previous one has already been applied! Therefore, you must ensure `old_text` doesn't reference text that has already been modified by a previous edit.
+- Don't explain the edits, just report them.
+- Only edit the file specified in `<file_to_edit>` and NEVER include edits to other files!
+- If you open an <old_text> tag, you MUST close it using </old_text>
+- If you open an <new_text> tag, you MUST close it using </new_text>
+
+<file_to_edit>
+{{path}}
+</file_to_edit>
+
+<edit_description>
+{{edit_description}}
+</edit_description>