Document null-equal joins in tree explain output

[2010YOUY01]

Summary

• drop the unused NullEquality::is_null_equals_null helper and inline the match checks in the join explain formatters
• extend the hash and sort-merge join explain renderers to use direct matches! checks for null equality annotations
• add a sqllogictest that captures the tree explain output, ensuring null-equal joins surface their annotation in that format
• refresh the tree explain sqllogictest to reflect real data source metrics and restore the session format back to indent after the tree assertions

Testing

• cargo test -p datafusion-sqllogictest --test sqllogictests -- join_is_not_distinct_from --complete

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https://chatgpt.com/codex/tasks/task_e_68cc0934ea0083219f573bce8a7d2f44