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_settings/src/assistant_settings.rs

@@ -6,7 +6,7 @@ use ::open_ai::Model as OpenAiModel;
 use anthropic::Model as AnthropicModel;
 use anyhow::{Result, bail};
 use deepseek::Model as DeepseekModel;
-use feature_flags::{Assistant2FeatureFlag, FeatureFlagAppExt};
+use feature_flags::{AgentStreamEditsFeatureFlag, Assistant2FeatureFlag, FeatureFlagAppExt};
 use gpui::{App, Pixels};
 use indexmap::IndexMap;
 use language_model::{CloudModel, LanguageModel};
@@ -87,9 +87,14 @@ pub struct AssistantSettings {
     pub profiles: IndexMap<AgentProfileId, AgentProfile>,
     pub always_allow_tool_actions: bool,
     pub notify_when_agent_waiting: NotifyWhenAgentWaiting,
+    pub stream_edits: bool,
 }
 
 impl AssistantSettings {
+    pub fn stream_edits(&self, cx: &App) -> bool {
+        cx.has_flag::<AgentStreamEditsFeatureFlag>() || self.stream_edits
+    }
+
     pub fn are_live_diffs_enabled(&self, cx: &App) -> bool {
         if cx.has_flag::<Assistant2FeatureFlag>() {
             return false;
@@ -218,6 +223,7 @@ impl AssistantSettingsContent {
                     profiles: None,
                     always_allow_tool_actions: None,
                     notify_when_agent_waiting: None,
+                    stream_edits: None,
                 },
                 VersionedAssistantSettingsContent::V2(ref settings) => settings.clone(),
             },
@@ -245,6 +251,7 @@ impl AssistantSettingsContent {
                 profiles: None,
                 always_allow_tool_actions: None,
                 notify_when_agent_waiting: None,
+                stream_edits: None,
             },
             None => AssistantSettingsContentV2::default(),
         }
@@ -495,6 +502,7 @@ impl Default for VersionedAssistantSettingsContent {
             profiles: None,
             always_allow_tool_actions: None,
             notify_when_agent_waiting: None,
+            stream_edits: None,
         })
     }
 }
@@ -550,6 +558,10 @@ pub struct AssistantSettingsContentV2 {
     ///
     /// Default: "primary_screen"
     notify_when_agent_waiting: Option<NotifyWhenAgentWaiting>,
+    /// Whether to stream edits from the agent as they are received.
+    ///
+    /// Default: false
+    stream_edits: Option<bool>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq)]
@@ -712,6 +724,7 @@ impl Settings for AssistantSettings {
                 &mut settings.notify_when_agent_waiting,
                 value.notify_when_agent_waiting,
             );
+            merge(&mut settings.stream_edits, value.stream_edits);
             merge(&mut settings.default_profile, value.default_profile);
 
             if let Some(profiles) = value.profiles {
@@ -843,6 +856,7 @@ mod tests {
                                 profiles: None,
                                 always_allow_tool_actions: None,
                                 notify_when_agent_waiting: None,
+                                stream_edits: None,
                             },
                         )),
                     }