Make idle_in_transaction regression test deterministic via per-row COPY delay

[Copilot]

DESCRIPTION: Add per-row sleep constraint to make idle_in_transaction shard move test deterministic

The regression test for the idle_in_transaction_session_timeout fix was ineffective: the 100-row shard copies in well under the 1 s timeout, so the bug would never be caught even without the fix.

Fix: Introduce a NOT VALID check constraint backed by a pg_sleep wrapper so PostgreSQL's COPY FROM (which fires check constraints per-row) makes the data-copy phase reliably exceed the timeout:

-- Created before distribute + insert, propagated to all workers by Citus DDL propagation
CREATE FUNCTION sleep_and_true(float8) RETURNS boolean LANGUAGE plpgsql AS $$
BEGIN
    PERFORM pg_sleep($1);
    RETURN true;
END;
$$;

-- NOT VALID: skips checking 100 existing rows; still fires for every row COPY writes
ALTER TABLE test_move ADD CONSTRAINT slow_copy
    CHECK (blocking_move_idle_timeout.sleep_and_true(0.1)) NOT VALID;

• NOT VALID keeps the initial INSERT fast while still enforcing the constraint on rows written by COPY during the shard move (~25 rows × 0.1 s ≈ 2.5 s per shard, well over the 1 s timeout).
• Schema-qualified function name ensures the deparsed ALTER TABLE propagated to workers resolves correctly regardless of worker search_path.
• DROP SCHEMA … CASCADE in cleanup also drops the function; expected output updated accordingly.

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