Associate each thread with a model (#29573)

This PR makes it possible to use different LLM models in the agent
panels of two different projects, simultaneously. It also properly
restores a thread's original model when restoring it from the history,
rather than having it use the default model. As before, newly-created
threads will use the current default model.

Release Notes:

- Enabled different project windows to use different models in the agent
panel
- Enhanced the agent panel so that when revisiting old threads, their
original model will be used.

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/agent/src/active_thread.rs

@@ -25,8 +25,8 @@ use gpui::{
 };
 use language::{Buffer, LanguageRegistry};
 use language_model::{
-    LanguageModelRegistry, LanguageModelRequestMessage, LanguageModelToolUseId, MessageContent,
-    RequestUsage, Role, StopReason,
+    LanguageModelRequestMessage, LanguageModelToolUseId, MessageContent, RequestUsage, Role,
+    StopReason,
 };
 use markdown::parser::{CodeBlockKind, CodeBlockMetadata};
 use markdown::{HeadingLevelStyles, Markdown, MarkdownElement, MarkdownStyle, ParsedMarkdown};
@@ -1252,7 +1252,7 @@ impl ActiveThread {
         cx.emit(ActiveThreadEvent::EditingMessageTokenCountChanged);
         state._update_token_count_task.take();
 
-        let Some(default_model) = LanguageModelRegistry::read_global(cx).default_model() else {
+        let Some(configured_model) = self.thread.read(cx).configured_model() else {
             state.last_estimated_token_count.take();
             return;
         };
@@ -1305,7 +1305,7 @@ impl ActiveThread {
                     temperature: None,
                 };
 
-                Some(default_model.model.count_tokens(request, cx))
+                Some(configured_model.model.count_tokens(request, cx))
             })? {
                 task.await?
             } else {
@@ -1338,7 +1338,7 @@ impl ActiveThread {
             return;
         };
         let edited_text = state.editor.read(cx).text(cx);
-        self.thread.update(cx, |thread, cx| {
+        let thread_model = self.thread.update(cx, |thread, cx| {
             thread.edit_message(
                 message_id,
                 Role::User,
@@ -1348,9 +1348,10 @@ impl ActiveThread {
             for message_id in self.messages_after(message_id) {
                 thread.delete_message(*message_id, cx);
             }
+            thread.get_or_init_configured_model(cx)
         });
 
-        let Some(model) = LanguageModelRegistry::read_global(cx).default_model() else {
+        let Some(model) = thread_model else {
             return;
         };