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/assistant_tools/src/assistant_tools.rs

@@ -42,7 +42,7 @@ use crate::list_directory_tool::ListDirectoryTool;
 use crate::now_tool::NowTool;
 use crate::thinking_tool::ThinkingTool;
 
-pub use edit_file_tool::EditFileToolInput;
+pub use edit_file_tool::{EditFileMode, EditFileToolInput};
 pub use find_path_tool::FindPathToolInput;
 pub use open_tool::OpenTool;
 pub use read_file_tool::{ReadFileTool, ReadFileToolInput};