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/eval/src/examples/mod.rs

@@ -12,12 +12,14 @@ use util::serde::default_true;
 use crate::example::{Example, ExampleContext, ExampleMetadata, JudgeAssertion};
 
 mod add_arg_to_trait_method;
+mod code_block_citations;
 mod file_search;
 
 pub fn all(examples_dir: &Path) -> Vec<Rc<dyn Example>> {
     let mut threads: Vec<Rc<dyn Example>> = vec![
         Rc::new(file_search::FileSearchExample),
         Rc::new(add_arg_to_trait_method::AddArgToTraitMethod),
+        Rc::new(code_block_citations::CodeBlockCitations),
     ];
 
     for example_path in list_declarative_examples(examples_dir).unwrap() {