[Feature] Add direct coverage for Parquet readers

[viiccwen]

ref: #1058

What

Add direct tests for qdp/qdp-core/src/readers/parquet.rs.

Current coverage for this file is still zero:

• Function coverage: 0.00% (0/31)
• Line coverage: 0.00% (0/415)

Why

We already have qdp/qdp-core/tests/parquet_io.rs, but those tests mainly exercise io.rs helper functions. The actual ParquetReader and ParquetStreamingReader implementations remain uncovered.

This is a large and important blind spot in the current Rust coverage report.

How

Add tests that instantiate the reader types directly and exercise both batch and streaming behavior.

Suggested coverage targets:

• ParquetReader::new() rejects missing files and bad schemas
• ParquetReader::read_batch() handles List<Float64>
• ParquetReader::read_batch() handles FixedSizeList<Float64>
• reject inconsistent sample sizes across rows
• ParquetStreamingReader::new() accepts scalar Float64 for basis encoding
• ParquetStreamingReader returns chunked data and sample-size metadata correctly
• assert empty-file or no-data behavior explicitly

This issue is done when readers/parquet.rs is no longer at 0% coverage and existing Parquet tests still pass.

[0lai0]

take