agent: Overwrite files more cautiously (#30649)

1. The `edit_file` tool tended to use `create_or_overwrite` a bit too
often, leading to corruption of long files. This change replaces the
boolean flag with an `EditFileMode` enum, which helps Agent make a more
deliberate choice when overwriting files.

With this change, the pass rate of the new eval increased from 10% to
100%.

2. eval: Added ability to run eval on top of an existing thread. Threads
can now be loaded from JSON files in the `SerializedThread` format,
which makes it easy to use real threads as starting points for
tests/evals.

3. Don't try to restore tool cards when running in headless or eval mode
-- we don't have a window to properly do this.

Release Notes:

- N/A

crates/agent/src/thread_store.rs

@@ -386,6 +386,25 @@ impl ThreadStore {
         })
     }
 
+    pub fn create_thread_from_serialized(
+        &mut self,
+        serialized: SerializedThread,
+        cx: &mut Context<Self>,
+    ) -> Entity<Thread> {
+        cx.new(|cx| {
+            Thread::deserialize(
+                ThreadId::new(),
+                serialized,
+                self.project.clone(),
+                self.tools.clone(),
+                self.prompt_builder.clone(),
+                self.project_context.clone(),
+                None,
+                cx,
+            )
+        })
+    }
+
     pub fn open_thread(
         &self,
         id: &ThreadId,
@@ -411,7 +430,7 @@ impl ThreadStore {
                         this.tools.clone(),
                         this.prompt_builder.clone(),
                         this.project_context.clone(),
-                        window,
+                        Some(window),
                         cx,
                     )
                 })