Lay the groundwork for a Rust-based eval (#28488)

Also, we moved the logic for driving the agentic loop into `Thread` so
that we don't have to re-implement it.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/agent/src/thread.rs

@@ -1181,7 +1181,8 @@ impl Thread {
                     match result.as_ref() {
                         Ok(stop_reason) => match stop_reason {
                             StopReason::ToolUse => {
-                                cx.emit(ThreadEvent::UsePendingTools);
+                                let tool_uses = thread.use_pending_tools(cx);
+                                cx.emit(ThreadEvent::UsePendingTools { tool_uses });
                             }
                             StopReason::EndTurn => {}
                             StopReason::MaxTokens => {}
@@ -1369,10 +1370,7 @@ impl Thread {
         )
     }
 
-    pub fn use_pending_tools(
-        &mut self,
-        cx: &mut Context<Self>,
-    ) -> impl IntoIterator<Item = PendingToolUse> + use<> {
+    pub fn use_pending_tools(&mut self, cx: &mut Context<Self>) -> Vec<PendingToolUse> {
         let request = self.to_completion_request(RequestKind::Chat, cx);
         let messages = Arc::new(request.messages);
         let pending_tool_uses = self
@@ -1460,18 +1458,36 @@ impl Thread {
                             output,
                             cx,
                         );
-
-                        cx.emit(ThreadEvent::ToolFinished {
-                            tool_use_id,
-                            pending_tool_use,
-                            canceled: false,
-                        });
+                        thread.tool_finished(tool_use_id, pending_tool_use, false, cx);
                     })
                     .ok();
             }
         })
     }
 
+    fn tool_finished(
+        &mut self,
+        tool_use_id: LanguageModelToolUseId,
+        pending_tool_use: Option<PendingToolUse>,
+        canceled: bool,
+        cx: &mut Context<Self>,
+    ) {
+        if self.all_tools_finished() {
+            let model_registry = LanguageModelRegistry::read_global(cx);
+            if let Some(ConfiguredModel { model, .. }) = model_registry.default_model() {
+                self.attach_tool_results(cx);
+                if !canceled {
+                    self.send_to_model(model, RequestKind::Chat, cx);
+                }
+            }
+        }
+
+        cx.emit(ThreadEvent::ToolFinished {
+            tool_use_id,
+            pending_tool_use,
+        });
+    }
+
     pub fn attach_tool_results(&mut self, cx: &mut Context<Self>) {
         // Insert a user message to contain the tool results.
         self.insert_user_message(
@@ -1495,11 +1511,12 @@ impl Thread {
             let mut canceled = false;
             for pending_tool_use in self.tool_use.cancel_pending() {
                 canceled = true;
-                cx.emit(ThreadEvent::ToolFinished {
-                    tool_use_id: pending_tool_use.id.clone(),
-                    pending_tool_use: Some(pending_tool_use),
-                    canceled: true,
-                });
+                self.tool_finished(
+                    pending_tool_use.id.clone(),
+                    Some(pending_tool_use),
+                    true,
+                    cx,
+                );
             }
             canceled
         };
@@ -1866,12 +1883,7 @@ impl Thread {
 
         self.tool_use
             .insert_tool_output(tool_use_id.clone(), tool_name, err, cx);
-
-        cx.emit(ThreadEvent::ToolFinished {
-            tool_use_id,
-            pending_tool_use: None,
-            canceled: true,
-        });
+        self.tool_finished(tool_use_id.clone(), None, true, cx);
     }
 }
 
@@ -1897,14 +1909,14 @@ pub enum ThreadEvent {
     MessageDeleted(MessageId),
     SummaryGenerated,
     SummaryChanged,
-    UsePendingTools,
+    UsePendingTools {
+        tool_uses: Vec<PendingToolUse>,
+    },
     ToolFinished {
         #[allow(unused)]
         tool_use_id: LanguageModelToolUseId,
         /// The pending tool use that corresponds to this tool.
         pending_tool_use: Option<PendingToolUse>,
-        /// Whether the tool was canceled by the user.
-        canceled: bool,
     },
     CheckpointChanged,
     ToolConfirmationNeeded,