Remove vendored PushDownLeafProjections workaround after DataFusion upgrade

[paleolimbot]

Problem

SedonaDB currently depends on DataFusion 54.1.0, which contains a logical optimizer correctness bug in PushDownLeafProjections when nested-field access is applied to a struct produced by Unnest.

For a plan shaped like:

Projection: get_field(dump, "geom")
  Unnest: dump
    Projection: ST_Dump(geometry) AS dump

the optimizer can push get_field(dump, "geom") below Unnest. At that point dump is still List<Struct<...>>, rather than the post-unnest Struct<...>, and optimization fails with:

Optimizer rule 'push_down_leaf_projections' failed
caused by
Execution error: Cannot access field at argument 1: type List(Struct(...))
is not Struct, Map, or Null

This is a plan-correctness bug, not a security issue.

Upstream status

The bug was reported and fixed upstream:

• PushDownLeafProjections error with Unnest nodes datafusion#22615 describes PushDownLeafProjections crossing Unnest, including the same-named pre/post-unnest column problem.
• fix: make PushDownLeafProjections work with unnest datafusion#22620 fixes it by treating Unnest as a semantic barrier for leaf-projection pushdown.
• Make LogicalPlan::Unnest expression/rebuild contracts consistent datafusion#22783 subsequently makes the LogicalPlan::Unnest expression/rebuild contracts consistent while retaining the semantic barrier.

The reduced reproducer fails with DataFusion 54.1.0 and succeeds on DataFusion main at e1942b144.

Temporary SedonaDB workaround

Until SedonaDB upgrades to a DataFusion release containing apache/datafusion#22620, sedona-query-planner replaces DataFusion's optimizer rule named push_down_leaf_projections with a compatibility rule that prevents leaf projection pushdown across Unnest.

Regression coverage includes both a DataFusion-only List<Struct> plan and SedonaDB's ST_Dump → unnest → geom workflow.

Action item

• After upgrading DataFusion to a release containing fix: make PushDownLeafProjections work with unnest datafusion#22620, remove the vendored PushDownLeafProjections compatibility rule and its registration, while retaining appropriate regression coverage against the upstream rule.