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/language_model/src/registry.rs

@@ -18,6 +18,7 @@ impl Global for GlobalLanguageModelRegistry {}
 #[derive(Default)]
 pub struct LanguageModelRegistry {
     active_model: Option<ActiveModel>,
+    editor_model: Option<ActiveModel>,
     providers: BTreeMap<LanguageModelProviderId, Arc<dyn LanguageModelProvider>>,
     inline_alternatives: Vec<Arc<dyn LanguageModel>>,
 }
@@ -29,6 +30,7 @@ pub struct ActiveModel {
 
 pub enum Event {
     ActiveModelChanged,
+    EditorModelChanged,
     ProviderStateChanged,
     AddedProvider(LanguageModelProviderId),
     RemovedProvider(LanguageModelProviderId),
@@ -128,6 +130,22 @@ impl LanguageModelRegistry {
         }
     }
 
+    pub fn select_editor_model(
+        &mut self,
+        provider: &LanguageModelProviderId,
+        model_id: &LanguageModelId,
+        cx: &mut Context<Self>,
+    ) {
+        let Some(provider) = self.provider(provider) else {
+            return;
+        };
+
+        let models = provider.provided_models(cx);
+        if let Some(model) = models.iter().find(|model| &model.id() == model_id).cloned() {
+            self.set_editor_model(Some(model), cx);
+        }
+    }
+
     pub fn set_active_provider(
         &mut self,
         provider: Option<Arc<dyn LanguageModelProvider>>,
@@ -162,6 +180,28 @@ impl LanguageModelRegistry {
         }
     }
 
+    pub fn set_editor_model(
+        &mut self,
+        model: Option<Arc<dyn LanguageModel>>,
+        cx: &mut Context<Self>,
+    ) {
+        if let Some(model) = model {
+            let provider_id = model.provider_id();
+            if let Some(provider) = self.providers.get(&provider_id).cloned() {
+                self.editor_model = Some(ActiveModel {
+                    provider,
+                    model: Some(model),
+                });
+                cx.emit(Event::EditorModelChanged);
+            } else {
+                log::warn!("Active model's provider not found in registry");
+            }
+        } else {
+            self.editor_model = None;
+            cx.emit(Event::EditorModelChanged);
+        }
+    }
+
     pub fn active_provider(&self) -> Option<Arc<dyn LanguageModelProvider>> {
         Some(self.active_model.as_ref()?.provider.clone())
     }
@@ -170,6 +210,10 @@ impl LanguageModelRegistry {
         self.active_model.as_ref()?.model.clone()
     }
 
+    pub fn editor_model(&self) -> Option<Arc<dyn LanguageModel>> {
+        self.editor_model.as_ref()?.model.clone()
+    }
+
     /// Selects and sets the inline alternatives for language models based on
     /// provider name and id.
     pub fn select_inline_alternative_models(