Adds a custom subquery decorrelation pass to Beam SQL to handle complex correlated subqueries.

The Problem

Calcite's default SqlToRelConverter decorrelates most queries, but some complex correlated shapes (like correlated EXISTS or IN subqueries inside PROJECT or JOIN conditions) survive as RexSubQuery or residual LogicalCorrelate nodes.

Beam's Volcano planner ruleset has no physical converter rules for a general LogicalCorrelate node. When these residues survive, the planner fails with a CannotPlanException.

The Fix

Implemented a pre-Volcano normalization pass in CalciteQueryPlanner.convertToBeamRel:

1. normalizeForVolcano: This pass runs a short-lived HepPlanner with Calcite's *SUB_QUERY_TO_CORRELATE rules to turn any un-expanded RexSubQuery into a LogicalCorrelate.
2. It then calls RelDecorrelator.decorrelateQuery to lower these correlates into standard relational shapes (Join, Aggregate, Project, Filter) which existing Beam rules already know how to handle.
3. This pass is strictly gated on the tree actually referencing a correlation variable (using RelOptUtil.getVariablesUsed), making it a no-op for shapes like UNNEST (LogicalCorrelate(_, Uncollect)) which define but do not reference a correlation variable, preserving them for BeamUnnestRule.
4. It runs before the Volcano planner and cost-based metadata provider swap, keeping it off the recursive cost path.

Added a detailed design document in decorrelation_design.md explaining the approach and the Volcano planner convergence safety.