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/language/src/language.rs

@@ -245,6 +245,10 @@ impl CachedLspAdapter {
         self.adapter.retain_old_diagnostic(previous_diagnostic, cx)
     }
 
+    pub fn underline_diagnostic(&self, diagnostic: &lsp::Diagnostic) -> bool {
+        self.adapter.underline_diagnostic(diagnostic)
+    }
+
     pub fn diagnostic_message_to_markdown(&self, message: &str) -> Option<String> {
         self.adapter.diagnostic_message_to_markdown(message)
     }
@@ -470,6 +474,16 @@ pub trait LspAdapter: 'static + Send + Sync {
         false
     }
 
+    /// Whether to underline a given diagnostic or not, when rendering in the editor.
+    ///
+    /// 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.
+    fn underline_diagnostic(&self, _diagnostic: &lsp::Diagnostic) -> bool {
+        true
+    }
+
     /// Post-processes completions provided by the language server.
     async fn process_completions(&self, _: &mut [lsp::CompletionItem]) {}