False positive no-required-schema-properties-undefined when required used inside not #3104

Describe the bug

no-required-schema-properties-undefined false positive: required inside not (absence assertions / mutual exclusion).

The rule flags required entries inside a not subschema as "Required property 'x' is not defined." But required under not is the standard JSON Schema idiom for asserting a property's absence (or mutual exclusion of properties) — there is intentionally nothing to declare in a sibling properties, and the properties may even be declared on the enclosing object schema. The rule's purpose (catching typos where required names an undeclared property) doesn't apply to these assertion-only subschemas.

To Reproduce
redocly.yaml

extends: []
rules:
  no-required-schema-properties-undefined: warn

openapi.yaml

openapi: 3.1.0
info:
  title: Repro
  version: 1.0.0
paths:
  /contacts:
    post:
      operationId: createContact
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Contact'
      responses:
        '201':
          description: Created.
components:
  schemas:
    Contact:
      type: object
      properties:
        email:
          type: string
        phone:
          type: string
      # email and phone are mutually exclusive
      not:
        required:
          - email
          - phone

Root cause: in no-required-schema-properties-undefined, findCompositionRoot walks up only through allOf/anyOf/oneOf parents (isCompositionChild), so any schema reached via not never resolves a composition root and its own (usually empty) properties is the only thing checked.

Actual behavior

#/components/schemas/Contact/not/required/0  Required property 'email' is not defined.
#/components/schemas/Contact/not/required/1  Required property 'phone' is not defined.

Expected behavior

No warnings. Either (a) required occurrences inside a not subschema (at any depth) are exempt, since they are presence/absence assertions rather than shape declarations, or (b) property resolution traverses the not boundary to the enclosing object schema the way it already traverses allOf/anyOf/oneOf.

Logs

OpenAPI description

Redocly version(s)

@redocly/cli 2.51.2

Node.js version(s)

OS, environment

Additional context