Make eval more resilient to bad input from LLM (#29703)

I saw a slice panic (for begin > end) in a debug build of the eval. This
should just be a failed assertion, not a panic that takes out the whole
eval run!

Release Notes:

- N/A

crates/eval/src/examples/code_block_citations.rs

@@ -98,8 +98,17 @@ impl Example for CodeBlockCitations {
 
                             if let Some(content_len) = content_len {
                                 // + 1 because there's a newline character after the citation.
-                                let content =
+                                let start_index = citation.len() + 1;
-                                    &text[(citation.len() + 1)..content_len - (citation.len() + 1)];
+                                let end_index = content_len.saturating_sub(start_index);
+
+                                if cx
+                                    .assert(
+                                        start_index <= end_index,
+                                        "Code block had a valid citation",
+                                    )
+                                    .is_ok()
+                                {
+                                    let content = &text[start_index..end_index];
 
                                     // deindent (trim the start of each line) because sometimes the model
                                     // chooses to deindent its code snippets for the sake of readability,
@@ -129,8 +138,8 @@ impl Example for CodeBlockCitations {
 
                                         if let Some(end_col) = range.end.col {
                                             let last_line = snippet.lines().last().unwrap();
-                                        snippet = snippet
+                                            snippet = snippet[..snippet.len() - last_line.len()
-                                            [..snippet.len() - last_line.len() + end_col as usize]
+                                                + end_col as usize]
                                                 .to_string();
                                         }
 
@@ -150,6 +159,7 @@ impl Example for CodeBlockCitations {
                                 }
                             }
                         }
+                    }
                 } else {
                     cx.assert(
                         false,