10.x: run text value set operations as SQL where order and repeats don't count (port of #485) #497

Problem

On main, once #485 merges (PR #493, stacked on #492), the text set operations run as SQL where order and repeats don't count: Intersect, Except and Add left unsorted, Count over them as count(DISTINCT …), and ==/!= as mutual containment. On release/10.x, the three run in memory, and Count, == and != over a text Union throw.

Change

After the #483 port, cherry-pick PR #493's commits with -x into a PR against release/10.x:

• The unguarded placeholders, the walker and expander changes in ValueSetSqlGuard, and the Count decline removed from ValueSetsMemberTranslator.
• The text sweep in CanonicalSetOperationParityTests, the flipped UnionCanonicalityTests, SetIntegrationTests and EfCore11RetargetTests cases, the 13 no-database tests, and the ARRAY arm of DeclaredNullabilityTests.
• The seeds text-set-count-counts-repeats and text-set-equality-compares-arrays, and union-equality-not-refused retired.
• The docs, AGENTS.md, claims E17–E20, E35 and E82, and translation-support.json.
• The changelog rows go under [10.4.0] in the root and EF Core changelogs.

Done when

On release/10.x, the tests pass on the EF Core 10.0.0 floor and on CI's newest-patch leg, on PostgreSQL 16 and 18, and both seeds fail their oracles.

Release

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