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),

crates/editor/src/display_map/fold_map.rs

@@ -1255,6 +1255,8 @@ pub struct Chunk<'a> {
     pub diagnostic_severity: Option<lsp::DiagnosticSeverity>,
     /// Whether this chunk of text is marked as unnecessary.
     pub is_unnecessary: bool,
+    /// Whether this chunk of text should be underlined.
+    pub underline: bool,
     /// Whether this chunk of text was originally a tab character.
     pub is_tab: bool,
     /// An optional recipe for how the chunk should be presented.
@@ -1422,6 +1424,7 @@ impl<'a> Iterator for FoldChunks<'a> {
                 diagnostic_severity: chunk.diagnostic_severity,
                 is_unnecessary: chunk.is_unnecessary,
                 is_tab: chunk.is_tab,
+                underline: chunk.underline,
                 renderer: None,
             });
         }