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).

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.