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/eval/src/instance.rs

@@ -1,4 +1,4 @@
-use agent::{Message, MessageSegment, ThreadStore};
+use agent::{Message, MessageSegment, SerializedThread, ThreadStore};
 use anyhow::{Context, Result, anyhow, bail};
 use assistant_tool::ToolWorkingSet;
 use client::proto::LspWorkProgress;
@@ -312,7 +312,14 @@ impl ExampleInstance {
             thread_store.update(cx, |thread_store, cx| thread_store.load_profile_by_id(profile_id, cx)).expect("Failed to load profile");
 
             let thread =
-                thread_store.update(cx, |thread_store, cx| thread_store.create_thread(cx))?;
+                thread_store.update(cx, |thread_store, cx| {
+                    if let Some(json) = &meta.existing_thread_json {
+                        let serialized = SerializedThread::from_json(json.as_bytes()).expect("Can't read serialized thread");
+                        thread_store.create_thread_from_serialized(serialized, cx)
+                    } else {
+                        thread_store.create_thread(cx)
+                    }
+                })?;
 
 
             thread.update(cx, |thread, _cx| {