feat: wire batch_size_bytes to Python and public Rust API

[westonpace]

Summary

Stacked on #6388. Please merge that PR first.

• Adds batch_size_bytes: Option<u64> to FileReaderOptions and propagates it through all 6 SchedulerDecoderConfig creation sites in the file reader
• Adds batch_size_bytes field + setter to Scanner, wired through both scan_fragments (via LanceScanConfig) and pushdown_scan (via FileReaderOptions in ScanConfig)
• Adds batch_size_bytes to LanceScanConfig, with try_new_v2 injecting it into FragReadConfig via FileReaderOptions
• Exposes batch_size_bytes in the Python API: LanceDataset.scanner(), to_table(), to_batches(), ScannerBuilder

Test plan

• cargo check -p lance-file -p lance --tests — clean
• cargo clippy -p lance-file -p lance --tests -- -D warnings — clean
• cargo fmt --all — applied
• cargo test -p lance-encoding -- byte_sized — 3/3 pass
• cargo test -p lance -- test_scan — 38/38 pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 56.06061% with 29 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/scanner.rs	39.28%	15 Missing and 2 partials ⚠️
rust/lance/src/io/exec/filtered_read.rs	41.66%	5 Missing and 2 partials ⚠️
rust/lance/src/io/exec/scan.rs	76.92%	1 Missing and 2 partials ⚠️
rust/lance-file/src/reader.rs	84.61%	2 Missing ⚠️

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