feat(reader): Add read_with_metrics() for scan I/O metrics

[mbutrovich]

Which issue does this PR close?

• Closes #.

What changes are included in this PR?

Add always-on per-scan I/O metrics to ArrowReader.

Motivation: Downstream engines need per-scan byte counts for their UIs. For example, DataFusion Comet uses this to populate bytes_scanned on its Iceberg scan operator, which flows through to Spark UI via TaskMetrics.inputMetrics.setBytesRead(). This must be per-scan, not global. Concurrent scans against the same FileIO need independent counters. The approach matches DataFusion's pattern of wrapping AsyncFileReader with a counting layer and is storage-backend agnostic.

ArrowReader::read() now returns ScanResult

• ScanResult wraps the record batch stream and ScanMetrics. Accessors: stream(), metrics(), into_parts().
• Metrics are always collected. One fetch_add(Relaxed) per I/O request, negligible overhead.
• Counter is created fresh per read() call, so cloned readers get independent metrics.

New file: crates/iceberg/src/arrow/scan_metrics.rs

• CountingFileRead<F: FileRead>: generic wrapper that increments a shared AtomicU64 on each read().
• ScanMetrics: public handle exposing bytes_read().
• ScanResult: public struct returned by ArrowReader::read().

FileRead blanket impl for Box<dyn FileRead>

• Enables generic CountingFileRead<F> to wrap the boxed reader returned by FileIO::reader().

Single open_parquet_file with counting

• All Parquet opens (data files and delete files) go through the same open_parquet_file wrapped with CountingFileRead, so bytes_read reflects total scan I/O.
• build_parquet_reader(): shared internals for reader construction and metadata loading.

FileScanTaskReader struct (refactor)

• Extracted process_file_scan_task's parameters into a Clone struct with a process(self, task) method, resolving a clippy::too_many_arguments violation. Struct and impl are co-located.

Re-exports

• ScanMetrics and ScanResult re-exported from iceberg::arrow and iceberg::scan.

Are these changes tested?

test_scan_metrics_bytes_read in reader.rs: asserts bytes_read() == 0 before stream consumption (the stream is lazy) and bytes_read() > 0 after. test_scan_metrics_includes_delete_file_bytes: reads the same data file with and without a positional delete file and asserts bytes_read is strictly greater when deletes are present. All existing reader and scan tests pass (updated to use ScanResult::stream()).

[mbutrovich]

@blackmwk I'm trying really hard not to add more to reader.rs. Let me know if you have any suggestions.

[CTTY]

Just took a quick pass, love the direction!

[blackmwk]

Thanks @mbutrovich for this pr!

[mbutrovich]

Thanks for the feedback @CTTY and @blackmwk! I will address the comments tomorrow.

[mbutrovich]

I believe I addressed all of the feedback. Thanks for the first pass @CTTY @blackmwk, please take another look whenever you get a spare moment. Thank you!