sql: include WHERE subquery inputs in lineage

[mattfaltyn]

closes: #4751

One-line summary for changelog:

SQL parser: include input tables referenced by WHERE predicate subqueries in table lineage.

Meaningful description

Problem

Select::visit traverses FROM relations, joins, and projections, but did not traverse the SELECT selection expression. As a result, queries such as:

SELECT 1 WHERE EXISTS (SELECT 1 FROM customers)

silently omitted customers from in_tables.

Solution

Visit the selection expression in its own context frame, then collect its table dependencies and aliases without merging predicate-only column ancestry into projected column lineage.

Add a focused table-lineage regression test for a WHERE EXISTS subquery. The existing nested-subquery column-lineage test continues to pass and guards against predicate columns leaking into output column lineage.

Impact

SQL parser consumers now receive complete input-table lineage for tables read only through WHERE predicate subqueries.

Validation

• cargo test from integration/sql: 143 passed, 3 ignored
• cargo fmt --all -- --check
• Exact Python-binding reproduction repeated twice for EXISTS and IN subqueries, with projection-subquery controls
• All configured hooks applicable to the changed Rust files
• Repository-wide hooks passed except Docker-backed ShellCheck and Go lint, which could not run because Docker is unavailable; the transient integration/common mypy setup failure passed on rerun

Checklist

• AI was used in creating this PR