Tidy up diagnostics more (#29629)

- Stop merging same row diagnostics
- (for Rust) show code fragments surrounded by `'s in monospace

Co-authored-by: Serge Radinovich <sergeradinovich@gmail.com>

Closes #29362

Release Notes:

- diagnostics: Diagnostics are no longer merged when they're on the same
line
- rust: Diagnostics now show code snippets in monospace font:

<img width="551" alt="Screenshot 2025-04-29 at 16 13 45"
src="https://github.com/user-attachments/assets/d289be31-717d-404f-a76a-a0cda3e96fbe"
/>

Co-authored-by: Serge Radinovich <sergeradinovich@gmail.com>

crates/language/src/language.rs

@@ -239,6 +239,10 @@ impl CachedLspAdapter {
             .process_diagnostics(params, server_id, existing_diagnostics)
     }
 
+    pub fn diagnostic_message_to_markdown(&self, message: &str) -> Option<String> {
+        self.adapter.diagnostic_message_to_markdown(message)
+    }
+
     pub async fn process_completions(&self, completion_items: &mut [lsp::CompletionItem]) {
         self.adapter.process_completions(completion_items).await
     }
@@ -460,6 +464,10 @@ pub trait LspAdapter: 'static + Send + Sync {
     /// Post-processes completions provided by the language server.
     async fn process_completions(&self, _: &mut [lsp::CompletionItem]) {}
 
+    fn diagnostic_message_to_markdown(&self, _message: &str) -> Option<String> {
+        None
+    }
+
     async fn labels_for_completions(
         self: Arc<Self>,
         completions: &[lsp::CompletionItem],