Add const validation keyword

[crmne]

Goal

Add support for JSON Schema const.

const means the value must equal one exact JSON value. It is similar to a one-value enum, but JSON Schema has a dedicated keyword for it.

Example JSON Schema:

{
  "const": "admin"
}

Proposed DSL

string :role, const: "admin"
integer :version, const: 1
boolean :enabled, const: true
null :deleted_at, const: nil

Acceptance criteria

• Works across primitive types.
• Output uses const.
• Behavior tests cover matching and non-matching values.
• Generated schemas validate against Draft 2020-12.

Related work

Generic enum support already has related work:

• Support runtime-evaluated schema values #27
• Feature request: Add enum support for integer type #30
• Add enum parameter to number and integer schema methods #29