Deep AND/OR predicate chains overflow protobuf recursion limit when the serialized plan is re-parsed #4526

Problem

Comet serializes And/Or as a left-deep BinaryExpr tree (one nesting level per operand). A query with many conjuncts/disjuncts -- e.g. a wide WHERE a AND b AND c AND ..., or generated data-skipping predicates -- produces a proto nested deeper than protobuf's default recursion limit (100). The overflow fires when the serialized Operator is re-parsed: on the JVM via OperatorOuterClass.Operator.parseFrom (e.g. findShuffleScanIndices, explain) and in the Rust prost decoder. The query fails even though Comet can evaluate it.

Why it's safe to rebalance

Comet evaluates And/Or vectorially -- both sides are always evaluated, with no row-level short-circuit -- so the chain is fully associative. Rebalancing the flattened operands into a depth-O(log n) tree is semantically identical; it only changes the proto's shape.

Proposed fix

Add QueryPlanSerde.flattenAssociative (flatten an associative chain to its leaves) and createBalancedBinaryExpr (rebuild as a balanced tree), and route CometAnd/CometOr through them.

Relationship to the Delta integration

This is a standalone correctness/robustness fix for any wide boolean predicate. It is surfaced by the in-progress Delta Lake contrib integration (Delta data-skipping builds deep conjunctions), so it would help to prioritize it alongside that work. A PR will follow shortly.