Reproduce UNION coercion bug

[meiji163]

Description

In release-19.0-github, a cross-shard UNION query with enum will fail with error coercion should not try to coerce this value: ENUM("b") (errno 1815) (sqlstate HY000)

go test -v -run 'TestUnion$' ./go/test/endtoend/vtgate/queries/union/...

Test:

CREATE TABLE t2(
  id3 bigint,
  id4 bigint, /* lookup vindex on id4 */
  e ENUM('a','b','c') DEFAULT 'a',
  primary key(id3)
) Engine=InnoDB;
INSERT INTO t2(id3, id4, e) VALUES (1,1,'a'), (2,2,'b'), (3,1,'a'), (4,2,'b');
(SELECT id3, e from t2 where e = 'a' and id4 = 1) union (SELECT e, id3 from t2 where e='b' and id4 = 2);

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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