Returns reader's row-group count in num_row_groups, dispatched by its
integer(int32)/integer(int64) kind (the int32 specific also error stops if the actual
count overflows int32 -- vanishingly unlikely in practice, but kept for consistency with
parquet_get_nrows's own int32/int64 overload). Reflects the file's physical layout;
unaffected by any filter= given to parquet_open_reader. See "Streaming/chunked reads" in
doc/pages/io/reading.md for the chunked-read loop this and parquet_get_chunk_size/
parquet_read_column_chunk are meant to be used together for.