Nodes that consume a file's trailing newline (such as markdown headings and metadata blocks) end at the start of a row past the last line. _get_nodes_by_line() indexed lines[end_row] unguarded and crashed with an IndexError, which the chunk generator masked by silently falling back to Pygments, so affected files never got tree-sitter highlighting. This change clamps such nodes to the end of the last line, and skips nodes that start past it entirely (error-recovery nodes can have odd extents). Testing Done: - Added unit tests for both clamping cases, and an end-to-end test for a newline-terminated markdown file with a YAML frontmatter injection. - Ran unit tests (reviewboard/treesitter).