assistant: Edit files tool (#26506)

Exposes a new "edit files" tool that the model can use to apply
modifications to files in the project. The main model provides
instructions and the tool uses a separate "editor" model (Claude 3.5 by
default) to generate search/replace blocks like Aider does:

````markdown
mathweb/flask/app.py
```python
<<<<<<< SEARCH
from flask import Flask
=======
import math
from flask import Flask
>>>>>>> REPLACE
```
````

The search/replace blocks are parsed and applied as they stream in. If a
block fails to parse, the tool will apply the other edits and report an
error pointing to the part of the input where it occurred. This should
allow the model to fix it.


Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant_settings/src/assistant_settings.rs

@@ -62,6 +62,7 @@ pub struct AssistantSettings {
     pub default_width: Pixels,
     pub default_height: Pixels,
     pub default_model: LanguageModelSelection,
+    pub editor_model: LanguageModelSelection,
     pub inline_alternatives: Vec<LanguageModelSelection>,
     pub using_outdated_settings_version: bool,
     pub enable_experimental_live_diffs: bool,
@@ -162,6 +163,7 @@ impl AssistantSettingsContent {
                                 })
                             }
                         }),
+                    editor_model: None,
                     inline_alternatives: None,
                     enable_experimental_live_diffs: None,
                 },
@@ -182,6 +184,7 @@ impl AssistantSettingsContent {
                         .id()
                         .to_string(),
                 }),
+                editor_model: None,
                 inline_alternatives: None,
                 enable_experimental_live_diffs: None,
             },
@@ -310,6 +313,7 @@ impl Default for VersionedAssistantSettingsContent {
             default_width: None,
             default_height: None,
             default_model: None,
+            editor_model: None,
             inline_alternatives: None,
             enable_experimental_live_diffs: None,
         })
@@ -340,6 +344,8 @@ pub struct AssistantSettingsContentV2 {
     default_height: Option<f32>,
     /// The default model to use when creating new chats.
     default_model: Option<LanguageModelSelection>,
+    /// The model to use when applying edits from the assistant.
+    editor_model: Option<LanguageModelSelection>,
     /// Additional models with which to generate alternatives when performing inline assists.
     inline_alternatives: Option<Vec<LanguageModelSelection>>,
     /// Enable experimental live diffs in the assistant panel.
@@ -470,6 +476,7 @@ impl Settings for AssistantSettings {
                 value.default_height.map(Into::into),
             );
             merge(&mut settings.default_model, value.default_model);
+            merge(&mut settings.editor_model, value.editor_model);
             merge(&mut settings.inline_alternatives, value.inline_alternatives);
             merge(
                 &mut settings.enable_experimental_live_diffs,
@@ -528,6 +535,10 @@ mod tests {
                                 provider: "test-provider".into(),
                                 model: "gpt-99".into(),
                             }),
+                            editor_model: Some(LanguageModelSelection {
+                                provider: "test-provider".into(),
+                                model: "gpt-99".into(),
+                            }),
                             inline_alternatives: None,
                             enabled: None,
                             button: None,