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/examples/overwrite_file.rs

@@ -0,0 +1,49 @@
+use anyhow::Result;
+use assistant_settings::AgentProfileId;
+use assistant_tools::{EditFileMode, EditFileToolInput};
+use async_trait::async_trait;
+
+use crate::example::{Example, ExampleContext, ExampleMetadata};
+
+pub struct FileOverwriteExample;
+
+/*
+This eval tests a fix for a destructive behavior of the `edit_file` tool.
+Previously, it would rewrite existing files too aggressively, which often
+resulted in content loss.
+
+Pass rate before the fix: 10%
+Pass rate after the fix:  100%
+*/
+
+#[async_trait(?Send)]
+impl Example for FileOverwriteExample {
+    fn meta(&self) -> ExampleMetadata {
+        let thread_json = include_str!("threads/overwrite-file.json");
+
+        ExampleMetadata {
+            name: "file_overwrite".to_string(),
+            url: "https://github.com/zed-industries/zed.git".to_string(),
+            revision: "023a60806a8cc82e73bd8d88e63b4b07fc7a0040".to_string(),
+            language_server: None,
+            max_assertions: Some(1),
+            profile_id: AgentProfileId::default(),
+            existing_thread_json: Some(thread_json.to_string()),
+        }
+    }
+
+    async fn conversation(&self, cx: &mut ExampleContext) -> Result<()> {
+        let response = cx.run_turns(1).await?;
+        let file_overwritten = if let Some(tool_use) = response.find_tool_call("edit_file") {
+            let input = tool_use.parse_input::<EditFileToolInput>()?;
+            match input.mode {
+                EditFileMode::Edit => false,
+                EditFileMode::Create | EditFileMode::Overwrite => true,
+            }
+        } else {
+            false
+        };
+
+        cx.assert(!file_overwritten, "File should be edited, not overwritten")
+    }
+}