[validator] Add built-in unique comparison validator constraint

[imdhemy]

We should add a built-in unique constraint under comparison constraints to validate that array elements are unique.

Proposal

Add a new built-in constraint named unique with options shaped like the existing built-ins, including shared validator metadata such as groups:

type UniqueOptions = ConstraintOptions & {
  message?: string;
  normalizer?: (value: unknown) => unknown;
  fields?: string[];
};

Example usage:

const rules = {
  tags: [{ unique: {} }],
  emails: [{ unique: { normalizer: value => typeof value === 'string' ? value.trim() : value } }],
  coordinates: [{ unique: { fields: ['latitude', 'longitude'] } }],
  usernames: [{ unique: { groups: ['create'] } }],
};

Expected behavior

• unique should live in the comparison constraint group.
• It should be exported from the validator constraints public API and registered in builtInConstraints.
• It should support shared validator metadata exposed through ConstraintOptions, including groups.
• It should validate arrays for duplicate elements.
• By default, comparison should be strict, so '7' and 7 are considered different values.
• It should return no violations when all array elements are unique.
• It should return one violation when at least one duplicate is found.
• It should support a custom message option, consistent with the existing constraints.
• normalizer should be applied to each array element before uniqueness is checked.
• fields should allow uniqueness checks based on a combination of fields for arrays of objects.
• It should preserve current optional-field semantics used by other constraints: undefined should be considered valid unless presence is enforced separately.
• null handling should be defined explicitly and covered by tests. [Failure, as null is not a list]
• undefined handling should be defined explicitly and covered by tests. [ByPass validation]
• Non-array values should be handled explicitly and covered by tests.

Default message

A sensible default would be:

This collection should contain only unique elements.

[EmanFateen]

Hi @imdhemy How should the constrain behave if the received value is null? I didn't get it from the expected behavior.

[imdhemy]

@EmanFateen I've updated the description and added a new item for undefined.

• null handling should be defined explicitly and covered by tests. [Failure, as null is not a list]
• undefined handling should be defined explicitly and covered by tests. [ByPass validation].

Also a list should contain a single null value to be unique.