Allow LSP adapters to decide, which diagnostics to underline (#31450)

Closes
https://github.com/zed-industries/zed/pull/31355#issuecomment-2910439798

<img width="1728" alt="image"
src="https://github.com/user-attachments/assets/2eaa8e9b-00bc-4e99-ac09-fceb2d932e41"
/>


Release Notes:

- N/A

crates/editor/src/display_map.rs

@@ -960,11 +960,8 @@ impl DisplaySnapshot {
             }) {
                 if chunk.is_unnecessary {
                     diagnostic_highlight.fade_out = Some(editor_style.unnecessary_code_fade);
-                // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticTag
-                // states that
-                // > Clients are allowed to render diagnostics with this tag faded out instead of having an error squiggle.
-                // for the unnecessary diagnostics, so do not underline them.
-                } else if editor_style.show_underlines {
+                }
+                if chunk.underline && editor_style.show_underlines {
                     let diagnostic_color = super::diagnostic_style(severity, &editor_style.status);
                     diagnostic_highlight.underline = Some(UnderlineStyle {
                         color: Some(diagnostic_color),