Support case-insensitive comparison with ObjectListFilter

[Copilot]

ObjectListFilter only supported case-sensitive string comparisons. This adds four case-insensitive variants using StringComparison.OrdinalIgnoreCase.

New filter cases

DU case	Operator	GraphQL suffix (long / short)
EqualsCI	===~	_equals_ci / _eqi
StartsWithCI	=@@~	_starts_with_ci / _swci
EndsWithCI	@@=~	_ends_with_ci / _ewci
ContainsCI	@=@~	_contains_ci / _cci

Usage

// F# directly
let filter = ContainsCI { FieldName = "firstName"; Value = "jon" }
queryable.Apply(filter)

// Via operators
open ObjectListFilter.Operators
let filter = "firstName" ===~ "jonathan"   // case-insensitive equals

// Via GraphQL input (coerced from query argument)
// { firstName_contains_ci: "jon" }
// { firstName_eqi: "jonathan" }

Implementation notes

• buildFilterExpr emits Expression.Call using the string.Equals/StartsWith/EndsWith/Contains(string, StringComparison) overloads
• CI suffix patterns are matched before their case-sensitive counterparts in parseFieldCondition to avoid ambiguous prefix matches (e.g. _contains_ci before _contains)