Thread `PhysicalOptimizerContext` through `join_selection` stats helpers (CBO extension pattern)

[asolimando]

Is your feature request related to a problem or challenge?

JoinSelection's internal stats helpers (get_stats, should_swap_join_order,
supports_collect_by_thresholds) take Option<&StatisticsRegistry> and
&ConfigOptions pre-extracted from the PhysicalOptimizerContext. Every new
session-scoped input added to the context for cost-based estimation forces a
signature change across all three helpers and their call sites.

Describe the solution you'd like

Pass &dyn PhysicalOptimizerContext to these helpers instead; they read
statistics_registry() / config_options() off it. New context inputs then need
no helper-signature changes.

This is also the pattern any future physical optimizer rule implementing CBO
should follow: take the PhysicalOptimizerContext and read what it needs off it,
rather than threading individual extracted inputs.

Describe alternatives you've considered

• Keep threading individual params — every new input churns all three signatures.
• Pass SessionState directly — not possible; datafusion-physical-optimizer
  can't depend on datafusion-core, which is why PhysicalOptimizerContext
  exists.

Additional context

Non-breaking, mechanical: the helpers are crate-internal (get_stats /
supports_collect_by_thresholds private, should_swap_join_order pub(crate)),
no cross-crate callers, public PhysicalOptimizerRule methods unchanged.

[asolimando]

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