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) {

crates/assistant_tools/src/streaming_edit_file_tool.rs

@@ -61,6 +61,9 @@ pub struct StreamingEditFileToolInput {
 
     /// If true, this tool will recreate the file from scratch.
     /// If false, this tool will produce granular edits to an existing file.
+    ///
+    /// When a file already exists or you just created it, always prefer editing
+    /// it as opposed to recreating it from scratch.
     pub create_or_overwrite: bool,
 }