Stabilize PG19 ORDER BY pushdown regression plan

[ihalatci]

Stabilize PostgreSQL 19 ORDER BY pushdown plan assertions ## Context PostgreSQL BUG #19517 caused eager aggregation on semi/antijoin inner relations to return incorrect results on 19beta1 (5|130). Upstream commit ffeda04259bb fixes that bug, and the same reproducer returns the correct 5|26 on 19beta2. This PR therefore does not disable enable_eager_aggregate or enable_hashagg globally or in the regression harness. The H4 case in multi_orderby_pushdown is independent of that correctness fix. On 19beta2, with eager aggregation at its default, it still chooses a worker Nested Loop plan instead of the asserted Sort -> Hash Semi Join shape. Both Citus Sorted Merge Adaptive nodes remain, so pushdown itself is intact. This change stabilizes only the H4 EXPLAIN by wrapping it in a transaction and propagating transaction-local planner settings: sql BEGIN; SET LOCAL citus.propagate_set_commands TO 'local'; SET LOCAL enable_nestloop TO off; -- existing H4 EXPLAIN COMMIT; The expected plan is otherwise unchanged and continues to assert sorted-merge pushdown and the worker hash semi-join shape. There are no production-code or regression-harness changes. ## Validation - PostgreSQL 17.10: -Werror build and focused multi_orderby_pushdown path, 15/15 passed. - PostgreSQL 18.4: -Werror build and focused path, 15/15 passed. - PostgreSQL 19beta2: -Werror build and focused path, 15/15 passed with default enable_eager_aggregate=on. - PostgreSQL 19beta2 upstream regression suite: 245/245 passed, including eager_aggregate. - PostgreSQL 19beta2 Citus semi/antijoin path: multi_subquery_in_where_reference_clause, 15/15 passed with eager aggregation enabled. - Perl syntax, style, and diff whitespace checks passed. Closes #8666 > This draft targets the non-default pg19-support branch. GitHub may defer automatic issue closure until the change reaches the default branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (cfb4063) to head (355419b).

Additional details and impacted files

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##           pg19-support    #8680   +/-   ##
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  Coverage         88.79%   88.80%           
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  Files               288      288           
  Lines             64416    64416           
  Branches           8101     8101           
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+ Hits              57201    57205    +4     
+ Misses             4877     4874    -3     
+ Partials           2338     2337    -1     

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