Declared-nullability check: each emitted SQL function's declaration against PostgreSQL and the complete translated expression (#360's static half) #380

Problem

#349 (lower/upper) and #360 (array_cat) were declarations that told EF Core a function is NULL exactly when an argument is, where PostgreSQL said otherwise. #362 describes the static half that catches that class: list what the translators emit and check each declaration. Checking pg_proc.proisstrict alone is not enough, because what EF Core reasons about is the whole translated expression: UpperBound() on a discrete range is upper(x) - 1, the factories are a CASE around a non-strict constructor, and a predicate is wrapped in x IS NOT NULL AND ….

Change

A test, DeclaredNullabilityTests (integration project), that translates every discovered range, RangeSet and value set member over a nullable column and a required one, walks the SQL tree EF Core hands the provider, and for each SqlFunctionExpression it finds compares the declared IsNullable and ArgumentsPropagateNullability with pg_proc.proisstrict for that function and argument types. It then evaluates the complete translated expression on the server for NULL and non-NULL inputs and checks that it is NULL exactly when EF Core's declaration says it can be. Disagreements go through Candidates.Verdict.

Done when

Registered in test/oracles/oracles.json and shown to catch bound-declared-null-only-for-a-null-argument.patch; #360's array_cat is reported under #360.