Code block evals (#29619)

Add a targeted eval for code block formatting, and revise the system
prompt accordingly.

### Eval before, n=8

<img width="728" alt="eval before"
src="https://github.com/user-attachments/assets/552b6146-3d26-4eaa-86f9-9fc36c0cadf2"
/>

### Eval after prompt change, n=8 (excluding the new evals, so just
testing the prompt change)

<img width="717" alt="eval after"
src="https://github.com/user-attachments/assets/c78c7a54-4c65-470c-b135-8691584cd73e"
/>

Release Notes:

- N/A

crates/markdown/src/markdown.rs

@@ -1,6 +1,8 @@
 pub mod parser;
 mod path_range;
 
+pub use path_range::{LineCol, PathWithRange};
+
 use std::borrow::Cow;
 use std::collections::HashSet;
 use std::iter;

crates/markdown/src/path_range.rs

@@ -32,6 +32,20 @@ impl LineCol {
 }
 
 impl PathWithRange {
+    // Note: We could try out this as an alternative, and see how it does on evals.
+    //
+    // The closest to a standard way of including a filename is this:
+    // ```rust filename="path/to/file.rs#42:43"
+    // ```
+    //
+    // or, alternatively,
+    // ```rust filename="path/to/file.rs" lines="42:43"
+    // ```
+    //
+    // Examples where it's used this way:
+    // - https://mdxjs.com/guides/syntax-highlighting/#syntax-highlighting-with-the-meta-field
+    // - https://docusaurus.io/docs/markdown-features/code-blocks
+    // - https://spec.commonmark.org/0.31.2/#example-143
     pub fn new(str: impl AsRef<str>) -> Self {
         let str = str.as_ref();
         // Sometimes the model will include a language at the start,