Encourage editing over re-creating a file from scratch (#29870)

I also introduced a new eval to prove the encouragement actually makes a
difference.

Release Notes:

- Improved agent behavior when streaming edits, encouraging it to
editing files as opposed to creating them from scratch

crates/assistant_tools/src/assistant_tools.rs

@@ -54,6 +54,7 @@ pub use edit_file_tool::{EditFileTool, EditFileToolInput};
 pub use find_path_tool::FindPathToolInput;
 pub use open_tool::OpenTool;
 pub use read_file_tool::ReadFileToolInput;
+pub use streaming_edit_file_tool::StreamingEditFileToolInput;
 pub use terminal_tool::TerminalTool;
 
 pub fn init(http_client: Arc<HttpClientWithUrl>, cx: &mut App) {