Preserve cursor position when resetting excerpts (#27850)

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/assistant2/src/inline_assistant.rs

@@ -1363,10 +1363,7 @@ impl InlineAssistant {
                     multi_buffer.update(cx, |multi_buffer, cx| {
                         multi_buffer.push_excerpts(
                             old_buffer.clone(),
-                            Some(ExcerptRange {
-                                context: buffer_start..buffer_end,
-                                primary: None,
-                            }),
+                            Some(ExcerptRange::new(buffer_start..buffer_end)),
                             cx,
                         );
                     });