Restore README ObjectListFilter declaration block

Co-authored-by: xperiandri <2365592+xperiandri@users.noreply.github.com>

Author: Copilot

README.md

@@ -397,14 +397,29 @@ query TestQuery {
 }
 ```
 
-This filter is mapped by the middleware into the `ObjectListFilter` discriminated union, with cases such as `And`, `Or`, `Not`, `Equals`, `GreaterThan`, `GreaterThanOrEqual`, `LessThan`, `LessThanOrEqual`, `In`, `StartsWith`, `EndsWith`, `Contains`, `OfTypes`, and `FilterField`.
-
-- The cases `Equals`, `StartsWith`, `EndsWith`, and `Contains` accept a comparer parameter in the public discriminated union.
-- `Equals` and `Contains` keep the non-generic `System.Collections.IComparer` because they also support non-string comparable values in the public discriminated union.
-- When the filtered field is a string and you construct the DU cases directly, pass a `StringComparer` instance in that comparer slot. `StringComparer` is accepted there because it also implements `System.Collections.IComparer`. Use `StringComparer.Ordinal` for case-sensitive matching and `StringComparer.OrdinalIgnoreCase` for case-insensitive matching.
-- The built-in case-sensitive operators (`===`, `=@@`, `@@=`, `@=@`) use the default comparer behavior.
-- `Contains` is also used for collection membership checks, so the wrapped value type inside `FieldFilter<_>` stays `System.IComparable` instead of being limited to `string`.
-- If you were previously constructing `Equals`, `StartsWith`, `EndsWith`, or `Contains` directly, update those call sites to provide the comparer argument explicitly.
+This filter is mapped by the middleware inside an `ObjectListFilter` definition:
+
+```fsharp
+type FieldFilter<'Val> =
+    { FieldName : string
+      Value : 'Val }
+
+type ObjectListFilter =
+    | And of ObjectListFilter * ObjectListFilter
+    | Or of ObjectListFilter * ObjectListFilter
+    | Not of ObjectListFilter
+    | Equals of Filter : FieldFilter<System.IComparable> * Comparer : System.Collections.IComparer
+    | GreaterThan of FieldFilter<System.IComparable>
+    | GreaterThanOrEqual of FieldFilter<System.IComparable>
+    | LessThan of FieldFilter<System.IComparable>
+    | LessThanOrEqual of FieldFilter<System.IComparable>
+    | In of FieldFilter<obj list>
+    | StartsWith of Filter : FieldFilter<string> * Comparer : System.StringComparer
+    | EndsWith of Filter : FieldFilter<string> * Comparer : System.StringComparer
+    | Contains of Filter : FieldFilter<System.IComparable> * Comparer : System.Collections.IComparer
+    | OfTypes of System.Type list
+    | FilterField of FieldFilter<ObjectListFilter>
+```
 
 And the value recovered by the filter in the query is usable in the `ResolveFieldContext` of the resolve function of the field. To easily access it, you can use the extension method `Filter`, which returns an `ObjectListFilter voption` (it does not have a value if the object doesn't implement a list with the middleware generic definition, or if the user didn't provide a filter input).