tests: add Parallel Hash Full/Right Join regression cases

cbdb_parallel.sql: add a new test block covering:
  - Parallel Hash Full Join (HashedWorkers FULL JOIN HashedWorkers
    produces HashedOJ with parallel_workers=2)
  - Parallel Hash Right Join (pj_t1 is 3x larger than pj_t2, so the
    planner hashes the smaller pj_t2 and probes with pj_t1; result
    locus HashedWorkers)
  - Correctness checks: count(*) matches serial execution
  - Locus propagation: HashedOJ(parallel) followed by INNER JOIN
    produces HashedOJ; followed by FULL JOIN produces HashedOJ

join_hash.sql/out: CBDB-specific adaptations for the upstream parallel
full join test -- disable parallel mode for tests that require serial
plans, fix SAVEPOINT inside a parallel worker context, and update
expected output to match CBDB plan shapes.

Author: avamingli