refactor: thread SubqueryContext explicitly through physical planning

Removes the SessionState.clone() + execution_props_mut() trick used by
DefaultPhysicalPlanner to register scalar-subquery state for expression
lowering. That side channel relied on stashing per-plan state in
ExecutionProps, which forced physical planning to hold a mutable
SessionState and blocked moving QueryPlanner / PhysicalPlanner to
&dyn Session.

Introduces SubqueryContext in datafusion-expr and threads it explicitly
through create_initial_plan_inner, task_helper, map_logical_node_to_physical,
and the standalone planning helpers. Adds *_with_subquery_context dual
entry points for create_physical_expr, create_physical_exprs,
create_physical_sort_expr(s), create_window_expr(_with_name), and
LoweredAggregateBuilder.with_subquery_context — original public
signatures are preserved and delegate with SubqueryContext::default(),
so downstream callers are unaffected.

Drops the unreleased subquery_indexes and subquery_results fields from
ExecutionProps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer