Validates that a collection contains a specific element using an Eq comparator.

Contains enforces a membership constraint of the form: "collection must contain X".

Membership is defined by an Eq instance, not by structural equality or Elixir's in operator.

Options

• :value (required) The element that must be present in the collection.

• :eq (optional) An equality comparator. Defaults to Funx.Eq.Protocol.

• :message (optional) A custom error message callback (value -> String.t()).

Semantics

• Succeeds if the collection is a list and contains the element under Eq.
• Fails if the collection does not contain the element or is not a list.
• Nothing values pass unchanged.
• Just values are unwrapped before validation.