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/diagnostics/src/diagnostics.rs

@@ -513,7 +513,7 @@ impl ProjectDiagnosticsEditor {
                                         buffer.clone(),
                                         [ExcerptRange {
                                             context: context_range.clone(),
-                                            primary: Some(range.clone()),
+                                            primary: range.clone(),
                                         }],
                                         cx,
                                     )

crates/diagnostics/src/diagnostics_tests.rs

@@ -913,7 +913,7 @@ fn get_diagnostics_excerpts(
                     path: buffer.file().unwrap().path().to_path_buf(),
                     range: ExcerptRange {
                         context: range.context.to_point(buffer),
-                        primary: range.primary.map(|range| range.to_point(buffer)),
+                        primary: range.primary.to_point(buffer),
                     },
                     group_id: usize::MAX,
                     primary: false,