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.rs

@@ -458,7 +458,7 @@ impl Thread {
         tools: Entity<ToolWorkingSet>,
         prompt_builder: Arc<PromptBuilder>,
         project_context: SharedProjectContext,
-        window: &mut Window,
+        window: Option<&mut Window>, // None in headless mode
         cx: &mut Context<Self>,
     ) -> Self {
         let next_message_id = MessageId(