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/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