Closes #ISSUE

The model often uses the wrong closing tag, or has spaces around the
closing tag name. This PR makes it so that opening tags are treated as
authoritative and any closing tag with the name `new_text` `old_text` or
`edits` is accepted based on depth. This has the additional benefit that
the parsing is more robust with contents that contain `new_text`
`old_text` or `edits. I.e. the following test passes

```rust
    #[test]
    fn test_extract_xml_edits_with_conflicting_content() {
        let input = indoc! {r#"
            <edits path="component.tsx">
            <old_text>
            <new_text></new_text>
            </old_text>
            <new_text>
            <old_text></old_text>
            </new_text>
            </edits>
        "#};

        let result = extract_xml_replacements(input).unwrap();
        assert_eq!(result.file_path, "component.tsx");
        assert_eq!(result.replacements.len(), 1);
        assert_eq!(result.replacements[0].0, "<new_text></new_text>");
        assert_eq!(result.replacements[0].1, "<old_text></old_text>");
    }
```

Release Notes:

- N/A *or* Added/Fixed/Improved ...