openapi3: Validate() does not enforce two spec MUSTs (unique required entries, unique tag names)

[reuvenharrison]

Spun out of #1188 (typed-error follow-up is in #1199). Two rules the OpenAPI / JSON Schema specs state as MUST aren't enforced by Validate() today:

1. Duplicate entries in a schema's required array. JSON Schema 2020-12 §6.5.3 (OpenAPI 3.1) and draft-04 (OpenAPI 3.0) both say the elements of required MUST be unique. Schema.validate doesn't check it, so this passes:

type: object
required: [id, id]
properties: { id: { type: string } }

2. Duplicate tag names in the document-root tags list. The OpenAPI Object spec says "Each tag name in the list MUST be unique." Tags.Validate validates each tag but never checks name uniqueness across the list, so this passes:

tags:
  - name: pet
  - name: pet

Both are real spec MUSTs, but enforcing them is a behavior change: docs that pass Validate() today would start failing. That's why I kept them out of #1188/#1199 (which only add scope and don't change what validates) and split them here.

@fenollp, are you up for making these hard Validate() errors, or would you rather not break existing docs over them? If you're good with it, I'll send a PR using the existing cluster pattern, something like DuplicateRequiredFieldError{Field} and DuplicateTagError{Name} (cf. DuplicateParameterError / DuplicateOperationIDError).

[fenollp]

are you up for making these hard Validate() errors

Yes