Substrait roundtrip loses pushed-down TableScan offsets and limits #26034

Describe the bug

Substrait roundtripping can change query results by dropping TableScan::skip and TableScan::fetch.

The producer converts a table scan into a ReadRel, which cannot represent either field. from_limit compensates for the scan offset only when its immediate input is a TableScan. A standalone scan, or a scan separated from its parent limit by a projection, loses the pushed-down offset.

To Reproduce

Register a single CSV file as t, with one integer column x containing 0, 1, 2, 3, 4, 5. Build and roundtrip a plan using the public APIs:

let source = provider_as_source(ctx.table_provider("t").await?);
let scan = TableScanBuilder::new("t", source)
    .with_skip(Some(2))
    .with_fetch(Some(2))
    .build()?;
let plan = LogicalPlan::TableScan(scan);
let proto = to_substrait_plan(&plan, &ctx.state())?;
let restored = from_substrait_plan(&ctx.state(), &proto).await?;
let before = DataFrame::new(ctx.state(), plan).collect().await?;
let after = DataFrame::new(ctx.state(), restored).collect().await?;

The original plan returns [2, 3]; the restored standalone scan returns all six rows. With Limit(0, 2) -> Projection(x) -> TableScan(skip=2, fetch=2), the restored plan returns [0, 1]. The direct Limit(0, 2) -> TableScan case happens to retain [2, 3] because of the parent-limit compensation.

An independently verified offset-only reproduction on 60477917bd9eadd1a299185db6a8a08ccae7a4c7 returns [2, 3, 4, 5] before roundtripping and [0, 1, 2, 3, 4, 5] afterward.

Expected behavior

Roundtripping preserves the scan's offset and row limit regardless of whether a parent limit exists or a projection separates the nodes.

Additional context

A fix can encode the scan's skip/fetch as a FetchRel wrapping its ReadRel, and remove the parent-limit offset compensation to avoid applying the scan offset twice. Regression coverage should compare executed results, including offset-only scans, offset plus fetch, projections, nested limits, and scans without an offset.