[CaffeinatedCoder]

Closes #483

Problem

PostgreSQL compares the elements of a text[] column with the column's collation, and memory compares them ordinally. Under a nondeterministic (case- or accent-insensitive) collation, A equals a. On a StringSet column with UseCollation("case_insensitive") holding {A}, reproduced with the 10.3.0 packages:

• Contains("a"), IsSubsetOf({a}), Overlaps({a}) and == {a} all matched, where memory says false;
• Select(p => p.Tags.Remove("a")) gave {};
• ExecuteUpdate(… Remove("a")) stored {}, deleting an element memory keeps.

Change

The option decided on #483 (reject at model build), after measuring the other one:

• Rejected: COLLATE "C" in the operators. The GIN index carries the column's collation, so a collated operand turns every indexed @> into a sequential scan, on an ordinary en_US column too. The plans on PostgreSQL 16 and 18 are on the issue.
• StringSetCollationConvention (new, IModelFinalizingConvention, added through IConventionSetPlugin, which is additive and replaces no service). Building the model throws for a collation on a StringSet/StringSet<T> column other than C, POSIX, ucs_basic, pg_c_utf8 and default. Those compare bytes; a name doesn't say whether a collation is deterministic. A string set inside a JSON document is skipped, since the document holds its literal. A compiled model is checked when it is generated.
• ValueSetSqlGuard refuses EF.Functions.Collate over a string set with any other collation, on the final SQL tree, so a predicate, an equality, a projection and an ExecuteUpdate setter are all covered.
• Docs: a Good to know entry in Store a value set in an array column; Migration from v10.x gets a bullet and the section Check string set columns with a collation; the changelogs (root, EF Core) get a line under ### Breaking in 11.0.0-preview.1. The 10.x port adds ## Changed answers in 10.4.0.
• Oracles: claim E86.

It changes answers, so labelled silent: on 10.x it ships as the 10.4.0 minor under ### Changed answers.

Tests

• StringSetCollationTests (no database). A nondeterministic collation is rejected on StringSet and StringSet<T>, with a message naming the property. The five byte-order collations map with COLLATE. A JSON document isn't checked. EF.Functions.Collate is refused in a filter, an equality and a projection, and allowed with "C".
• StringSetCollationIntegrationTests (new). A column collated "C" answers Contains, Overlaps, Remove and ExecuteUpdate with Remove as memory does.
• Proved by reverting: without the plugin registration the two model-build tests fail, and without the guard case the Collate test fails.
• Full suite green: unit, conventions, ASP.NET Core and EF no-database projects, and both integration projects on PostgreSQL 16 and 18.

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