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,