Move large LogicalPlan arms out of DefaultPhysicalPlanner::map_logical_node_to_physical #26300

Is your feature request related to a problem or challenge?

DefaultPhysicalPlanner::map_logical_node_to_physical (physical_planner.rs#L593-L1978) is a single 1,203-line match over LogicalPlan, with cognitive complexity 202 and cyclomatic complexity 122. physical_planner.rs has had 80 commits in the last 12 months, so concurrent planner work regularly conflicts in this one function.

The match over variants is natural, but many arms carry large inline bodies:

• Join (about L1372-L1844, ~470 lines, selecting between hash, sort-merge, nested-loop and cross joins)
• Aggregate (about L1010-L1197)
• Copy and the five Dml arms (about L693-L932)
• Window

Some arms already delegate to helpers (e.g. Projection, Explain, Analyze), so the pattern is established.

Describe the solution you'd like

Keep the top-level match as a one-line-per-arm dispatcher and move each large arm into a private method on DefaultPhysicalPlanner: plan_join (further split by join algorithm), plan_aggregate, plan_window, plan_copy, plan_dml (one method for the five Dml arms), and so on. Do it incrementally, one arm per PR, starting with Join and Aggregate. Optionally move them into submodules (physical_planner/join.rs, aggregate.rs, dml.rs).

Describe alternatives you've considered

Leave it as is, at the cost of continued merge conflicts and harder review of planner changes.

Additional context

Line references are pinned to commit 0d79252.