V2 - make update/remove/copy mutually exclusive

not-remove-with-copy.yaml
not-update-with-copy.yaml
not-update-with-remove.yaml

The v1 specification allows update/copy/remove to be in the same action definition even though it's semantically meaningless. It also defines "an order of precedence" of each keyword, which is going to become more difficult to maintain, and understand by readers, as we introduce more keywords.

I propose that we make those mutually exclusive in the language, but also in the schema. Here is the salvaged schema update from my pull request.

  action-object:
    properties:
      target:
        type: string
        pattern: ^\$
      description:
        type: string
    oneOf:
      - properties:
          update:
            type:
              - string
              - boolean
              - object
              - array
              - number
              - "null"
        required:
          - update
      - properties:
          copy:
            type: string
        required:
          - copy
      - properties:
          remove:
            type: boolean
            default: false
        required:
          - remove
    required:
      - target
    $ref: "#/$defs/specification-extensions"
    unevaluatedProperties: false
  specification-extensions:
    patternProperties:
      ^x-: true

Originally posted by @baywet in https://github.com/OAI/Overlay-Specification/pull/150#discussion_r2448832872

Start by reviewing the proposed action-object schema and the three attached YAML examples for update/copy/remove combinations. Trace where the v1 language and schema are defined, then verify that each action permits only one of these keywords and that the examples are rejected appropriately.