update LanceNamespace::get_table_stats() to use manifests instead of file metadata

We have two paths to calculate "the size of a table":

1. lance_table::format::Manifest::summary().total_files_size, which uses each file's file_size_bytes (recorded in the manifest, basically free)
2. lance::dataset::statistics::DatasetStatisticsExt::calculate_data_stats() which is what feeds LanceDB's TableStatistics.total_bytes, which reads metadata from each file.

Path 1 is O(1) and Path 2 is O(data files). But Path 2 also calculates per-field stats. So if you need stats for each field, it's the only way to do it, but otherwise callers should use the manifest. One caller that still uses Path 2 but just sums up each field after is https://github.com/lance-format/lance/blob/acd623558faf642094818a63f37a2eafa9756a4d/rust/lance-namespace-impls/src/dir.rs#L4826-L4840, we should move that to use the manifest instead.

Start in rust/lance-namespace-impls/src/dir.rs at the table-statistics caller around lines 4826-4840. Compare its use of DatasetStatisticsExt::calculate_data_stats() with lance_table::format::Manifest::summary().total_files_size. Done means this caller obtains total table bytes from the manifest while retaining per-field statistics only where they are needed.