10.x: follow a text set operation through a scalar subquery in the guard's rewrite (port of #505) #510

Problem

On main, PR #508 makes the guard's rewrite follow a text set operation through a scalar subquery (#505). On release/10.x, ….Select(o => o.Tags.Union(x)).FirstOrDefault() == target compares the unsorted array_cat. 10.3.0 threw for that query, so this is a regression in the unreleased 10.4.0.

Change

Cherry-pick PR #508's commits with -x into a PR against release/10.x: the IsDerived arm, the two UnionCanonicalityTests, and the seed.

• 10.x specifics: no ### Changed answers row, because no release shipped the wrong answer. 10.3.0 threw, and 10.x: run text value set operations as SQL where order and repeats don't count (port of #485) #497's 10.4.0 row already promises == and Count. So the PR carries bug and severity/medium without silent.

Done when

On release/10.x, the tests pass on the EF Core 10.0.0 floor and on CI's newest-patch leg, they fail with the arm reverted, and the seed is caught.

Release

10.4.0. Fixed only on release/10.x, so close it by hand.