Query graph for Join reordering

[JanKaul]

Which issue does this PR close?

• Part of Consider adding a JoinGraph structure for representing joins #17719
• Part of Join Graph #18250

Rationale for this change

Join-ordering algorithms (DPhyp, DPccp, …) operate on a graph view of the join region rather than a LogicalPlan tree. DataFusion has no such structure today, so any future reordering rule has to re-derive one. This PR adds the data structure and the LogicalPlan ⇄ JoinGraph boundary so the follow-up enumeration work in epic #18249 has something concrete to build on.

What changes are included in this PR?

New datafusion/optimizer/src/reorder_join/join_graph.rs:

• JoinGraph, Node, Edge with NodeId / EdgeId handles backed by an internal VecMap (stable indices, no reuse on removal).
• JoinGraph::try_from_logical_plan(plan) -> Result<(JoinGraph, Vec<LogicalPlan>)>:
  • strips wrapper operators above the topmost join and returns them so the caller can reapply them after reordering;
  • decomposes inner joins into nodes (leaf relations) and edges (equi-join predicates);
  • hoists non-equi predicates — both Join.filter and Filter nodes sitting between inner joins — into a side-channel filters list;
  • treats non-inner joins and other operators nested between joins (Aggregate, Projection, …) as opaque leaves.
• reconstruct_plan(join_plan, wrappers) re-applies the stripped wrappers after reordering.
• Mutation API for the future enumerator: add_node, add_node_with_edge, remove_node, remove_edge,
  Node::neighbours, Node::connections.
• Module exported from datafusion/optimizer/src/lib.rs.

No optimizer rule is registered; nothing consumes JoinGraph outside tests.

Are these changes tested?

Yes — unit tests in join_graph.rs:

• three-way inner join with a non-equi Join.filter (predicate lands in side-channel);
• Filter between two inner joins (hoisted; both joins still decompose);
• Aggregate between two inner joins (opaque leaf);
• LEFT join nested inside an inner chain (opaque leaf);
• top-level non-inner join (single opaque leaf).

No sqllogictest changes — no planner-visible behavior yet.

Are there any user-facing changes?

No. JoinGraph is a new internal data structure in datafusion-optimizer; no existing API changes and no rule consumes it yet.