Fix horizontal scroll caused by diagnostic block width error (#19856)

Previously, when scrolling the diagnostics view with the mouse, we'd get
a spurious horizontal scroll (even if the content was not overflowing
horizontally) due to an error in the widths of the diagnostic blocks.

Release Notes:

- Fixed an issue where the project diagnostics view spuriously allowed
horizontal scrolling by a small amount.

crates/editor/src/editor.rs

@@ -14188,7 +14188,7 @@ pub fn diagnostic_block_renderer(
             .relative()
             .size_full()
             .pl(cx.gutter_dimensions.width)
-            .w(cx.max_width + cx.gutter_dimensions.width)
+            .w(cx.max_width - cx.gutter_dimensions.full_width())
             .child(
                 div()
                     .flex()