Function diff_file_symbols

Compute semantic diff for a single file by comparing symbols between two versions and checking which symbols overlap with changed hunks.

(
    base_content: Option<&str>,
    head_content: Option<&str>,
    file_path: &str,
    language: &str,
    hunks: &[(usize, usize)], // (start_line, line_count) of changed regions in head
)