Add default validation for unique server names

[ayushshrivastv]

Fixes #467

Summary

• add a default-on builtin validation that OAS 3.2 server names are unique across the document
• include root, path item, operation, and webhook server scopes in the uniqueness check
• update builtin validator tests for the default validation list and duplicate/unique server name cases

Testing

• git diff --check
• attempted swift test --filter BuiltinValidationTests, but SwiftPM dependency fetches timed out in this environment