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/buffer.rs

@@ -213,6 +213,8 @@ pub struct Diagnostic {
     pub severity: DiagnosticSeverity,
     /// The human-readable message associated with this diagnostic.
     pub message: String,
+    /// The human-readable message (in markdown format)
+    pub markdown: Option<String>,
     /// An id that identifies the group to which this diagnostic belongs.
     ///
     /// When a language server produces a diagnostic with
@@ -4616,6 +4618,7 @@ impl Default for Diagnostic {
             code_description: None,
             severity: DiagnosticSeverity::ERROR,
             message: Default::default(),
+            markdown: None,
             group_id: 0,
             is_primary: false,
             is_disk_based: false,

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],

crates/language/src/proto.rs

@@ -203,6 +203,7 @@ pub fn serialize_diagnostics<'a>(
             start: Some(serialize_anchor(&entry.range.start)),
             end: Some(serialize_anchor(&entry.range.end)),
             message: entry.diagnostic.message.clone(),
+            markdown: entry.diagnostic.markdown.clone(),
             severity: match entry.diagnostic.severity {
                 DiagnosticSeverity::ERROR => proto::diagnostic::Severity::Error,
                 DiagnosticSeverity::WARNING => proto::diagnostic::Severity::Warning,
@@ -422,6 +423,7 @@ pub fn deserialize_diagnostics(
                         proto::diagnostic::Severity::None => return None,
                     },
                     message: diagnostic.message,
+                    markdown: diagnostic.markdown,
                     group_id: diagnostic.group_id as usize,
                     code: diagnostic.code.map(lsp::NumberOrString::from_string),
                     code_description: diagnostic