ep: Fix code block extraction to require closing fence at line start (#46270)

`extract_last_codeblock` was using `find()` to locate closing fences,
which would match backticks anywhere in the text. This caused incorrect
parsing when the content contained inline backticks or nested code
blocks, resulting in wrong diffs.

Fix by requiring the closing fence to be preceded by a newline.
Release Notes:

- N/A

crates/edit_prediction_cli/src/format_prompt.rs

@@ -130,12 +130,22 @@ impl TeacherPrompt {
             .context("`buffer` should be filled in in the context collection step")?
             .content;
 
-        // Extract updated (new) editable region from the model response
+        // Extract updated (new) editable region from the model response.
+        // The model may include editable region markers in its output, so we need to strip them.
         let new_editable_region = extract_last_codeblock(response);
+        let mut new_editable_region = Self::extract_editable_region(&new_editable_region);
 
         // Reconstruct old editable region we sent to the model
         let old_editable_region = Self::format_editable_region(example);
         let old_editable_region = Self::extract_editable_region(&old_editable_region);
+
+        // Normalize leading newlines: if old starts with newline but new doesn't,
+        // prepend newline to new to preserve whitespace structure.
+        // This handles the case where the model drops the leading blank line.
+        if old_editable_region.starts_with('\n') && !new_editable_region.starts_with('\n') {
+            new_editable_region.insert(0, '\n');
+        }
+
         ensure!(
             cursor_file.contains(&old_editable_region),
             "Something's wrong: editable_region is not found in the cursor file"
@@ -214,6 +224,7 @@ impl TeacherPrompt {
         let end = text.find(Self::EDITABLE_REGION_END).unwrap_or(text.len());
 
         let region = &text[start..end];
+        let region = region.strip_suffix('\n').unwrap_or(region);
 
         region.replace("<|user_cursor|>", "")
     }
@@ -242,16 +253,16 @@ fn extract_last_codeblock(text: &str) -> String {
         }
 
         let backtick_count = backtick_end - start;
-        let closing_backticks = "`".repeat(backtick_count);
+        let closing_pattern = format!("\n{}", "`".repeat(backtick_count));
 
         while backtick_end < bytes.len() && bytes[backtick_end] != b'\n' {
             backtick_end += 1;
         }
 
-        if let Some(end_pos) = text[backtick_end..].find(&closing_backticks) {
-            let code_block = &text[backtick_end + 1..backtick_end + end_pos];
+        if let Some(end_pos) = text[backtick_end..].find(&closing_pattern) {
+            let code_block = &text[backtick_end + 1..backtick_end + end_pos + 1];
             last_block = Some(code_block.to_string());
-            search_start = backtick_end + end_pos + backtick_count;
+            search_start = backtick_end + end_pos + closing_pattern.len();
         } else {
             break;
         }
@@ -281,6 +292,37 @@ mod tests {
         assert_eq!(last_block, "last block\n");
     }
 
+    #[test]
+    fn test_extract_codeblock_with_nested_fences() {
+        let text = indoc::indoc! {"
+            `````
+            content with ``` inline
+            and ```python nested
+            more content
+            `````
+            "};
+        let last_block = extract_last_codeblock(text);
+        assert_eq!(
+            last_block,
+            "content with ``` inline\nand ```python nested\nmore content\n"
+        );
+    }
+
+    #[test]
+    fn test_extract_codeblock_ignores_inline_backticks() {
+        let text = indoc::indoc! {"
+            `````
+            here is some `code` with inline backticks
+            and here```more```stuff
+            `````
+            "};
+        let last_block = extract_last_codeblock(text);
+        assert_eq!(
+            last_block,
+            "here is some `code` with inline backticks\nand here```more```stuff\n"
+        );
+    }
+
     #[test]
     fn test_extract_editable_region() {
         let text = indoc::indoc! {"
@@ -301,7 +343,78 @@ mod tests {
             indoc::indoc! {"
             one
             two three
+            "}
+        );
+    }
 
+    #[test]
+    fn test_extract_last_codeblock_nested_bibtex() {
+        let text = indoc::indoc! {r#"
+            Looking at the edit history, I can see that a Citation section was just added.
+
+            `````
+            ## Collaborations
+            Our mission is to create a 4D generative model.
+
+            ## Citation
+
+            If you found Unique3D helpful, please cite our report:
+            ```bibtex
+            @misc{wu2024unique3d,
+                  title={Unique3D},
+            }
+            ```
+            `````
+            "#};
+        let last_block = extract_last_codeblock(text);
+        assert_eq!(
+            last_block,
+            indoc::indoc! {r#"
+            ## Collaborations
+            Our mission is to create a 4D generative model.
+
+            ## Citation
+
+            If you found Unique3D helpful, please cite our report:
+            ```bibtex
+            @misc{wu2024unique3d,
+                  title={Unique3D},
+            }
+            ```
+            "#}
+        );
+    }
+
+    #[test]
+    fn test_extract_editable_region_no_markers() {
+        let text = indoc::indoc! {"
+            one
+            two three
+            "};
+        let parsed = TeacherPrompt::extract_editable_region(text);
+        assert_eq!(
+            parsed,
+            indoc::indoc! {"
+            one
+            two three"}
+        );
+    }
+
+    #[test]
+    fn test_extract_editable_region_strips_cursor_marker() {
+        let text = indoc::indoc! {"
+            <|editable_region_start|>
+            one
+            <|user_cursor|>two three
+
+            <|editable_region_end|>
+            "};
+        let parsed = TeacherPrompt::extract_editable_region(text);
+        assert_eq!(
+            parsed,
+            indoc::indoc! {"
+            one
+            two three
             "}
         );
     }