assistant: Make scripting a first-class concept instead of a tool (#26338)

This PR makes refactors the scripting functionality to be a first-class
concept of the assistant instead of a generic tool, which will allow us
to build a more customized experience.

- The tool prompt has been slightly tweaked and is now included as a
system message in all conversations. I'm getting decent results, but now
that it isn't in the tools framework, it will probably require more
refining.

- The model will now include an `<eval ...>` tag at the end of the
message with the script. We parse this tag incrementally as it streams
in so that we can indicate that we are generating a script before we see
the closing `</eval>` tag. Later, this will help us interpret the script
as it arrives also.

- Threads now hold a `ScriptSession` entity which manages the state of
all scripts (from parsing to exited) in a centralized way, and will
later collect all script operations so they can be displayed in the UI.

- `script_tool` has been renamed to `assistant_scripting` 

- Script source now opens in a regular read-only buffer  

Note: We still need to handle persistence properly

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/assistant2/src/assistant_panel.rs

@@ -166,22 +166,25 @@ impl AssistantPanel {
         let history_store =
             cx.new(|cx| HistoryStore::new(thread_store.clone(), context_store.clone(), cx));
 
+        let thread = cx.new(|cx| {
+            ActiveThread::new(
+                workspace.clone(),
+                thread.clone(),
+                thread_store.clone(),
+                language_registry.clone(),
+                window,
+                cx,
+            )
+        });
+
         Self {
             active_view: ActiveView::Thread,
             workspace,
             project: project.clone(),
             fs: fs.clone(),
-            language_registry: language_registry.clone(),
+            language_registry,
             thread_store: thread_store.clone(),
-            thread: cx.new(|cx| {
-                ActiveThread::new(
-                    thread.clone(),
-                    thread_store.clone(),
-                    language_registry,
-                    window,
-                    cx,
-                )
-            }),
+            thread,
             message_editor,
             context_store,
             context_editor: None,
@@ -239,6 +242,7 @@ impl AssistantPanel {
         self.active_view = ActiveView::Thread;
         self.thread = cx.new(|cx| {
             ActiveThread::new(
+                self.workspace.clone(),
                 thread.clone(),
                 self.thread_store.clone(),
                 self.language_registry.clone(),
@@ -372,6 +376,7 @@ impl AssistantPanel {
                 this.active_view = ActiveView::Thread;
                 this.thread = cx.new(|cx| {
                     ActiveThread::new(
+                        this.workspace.clone(),
                         thread.clone(),
                         this.thread_store.clone(),
                         this.language_registry.clone(),