Fix 6 OpenAPI spec validation errors

The spec fails validation with Redocly CLI due to structural issues:

1. OAuth2ClientJsonSigningKeyRequest: `discriminator` and `required` were
   incorrectly nested inside `properties` instead of at the schema level

2. EnhancedDynamicNetworkZone: `include` and `exclude` properties used
   array syntax (`- $ref:`) instead of direct reference syntax (`$ref:`)

3. OAuth2ClientJsonWebKeyECRequest: typo `nullabe` corrected to `nullable`

Validation command: `npx @redocly/cli lint api.yaml`

Before: 6 errors, 1218 warnings
After: 0 errors, 1199 warnings

Fixes #1672

Author: mraible