DataFusion derives functional dependencies from PRIMARY KEY / UNIQUE
constraints and from GROUP BY keys, and several optimizer rules consume
them. Coverage today is scattered through group_by.slt, which makes it hard
to tell which behaviors are correct and which are known bugs.

Add a dedicated test file with one section per consumer:

  1. ReplaceDistinctWithAggregate         (removing DISTINCT)
  2. eliminate_duplicated_expr            (dropping trailing ORDER BY keys)
  3. optimize_projections                 (dropping GROUP BY expressions)
  4. add_group_by_exprs_from_dependencies (selecting non-grouped columns)
  5. GROUP BY derived keys on the NULL-padded side of an outer join

Each section contrasts a non-nullable PRIMARY KEY against a nullable UNIQUE
column, since that is the distinction the consumers get wrong: SQL UNIQUE
permits multiple NULL rows, so a UNIQUE column is a key only among the
non-NULL rows.

The expected results record current behavior. Four cases produce wrong
answers today and are labelled BUG with the expected result and a link to
the issue tracking it:

  * 1.2 DISTINCT over a nullable UNIQUE column returns both NULL rows (apache#23634)
  * 2.2 ORDER BY x, y drops the `y` key, so the NULL rows come back
    unordered (apache#23818)
  * 3.2 GROUP BY x, y drops `y`, merging the two NULL groups and losing a
    row (apache#23819)
  * 4.2 SELECT x, y ... GROUP BY x returns two rows for the x = NULL
    group (apache#23820)

Only logical plans are shown, since all of these rules run during logical
optimization.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>