Preserve row counts for zero-column tables in IO readers

[madsbk]

Follow-up to the zero-column support #22765.

The core PR made zero-column tables (shape (N, 0)) representable and threaded their row counts through table-producing algorithms, interop, and plc.Table. The remaining gap is in the IO readers, which still collapse zero-column reads to (0, 0) and lose the row count.

To do

• Parquet reader (including chunked and filtered read paths)
• ORC reader (including chunked read paths)
• CSV reader
• AVRO, JSON, and experimental Parquet paths (hybrid scan, deletion vectors) where applicable
• Unskip the placeholder reader tests (ParquetReaderTest, OrcReaderTest, and CsvReaderTest.ZeroColumnsPreservesRowCount)
• Remove the cudf-polars workarounds: DataFrame._num_rows_override, the DataFrameScan zero-width path, Scan._get_parquet_row_count_from_metadata, and the related xfails in inject_gpu_engine.py