Emit required properties and normalize nullable handling for OpenAPI 3.1

[tom-ioki]

This pull request enhances the OpenAPI schema generation in LightweightSerializer::Documentation by introducing new options for more accurate and standards-compliant output. It adds the ability to emit a required array for non-conditional properties and to optionally omit the deprecated nullable: true keyword, aligning with OpenAPI 3.1 best practices.

OpenAPI schema improvements:

• Added emit_required and omit_deprecated_nullable parameters to the openapi_schema method, allowing consumers to opt-in to emitting a required array for unconditional properties and to omit the deprecated nullable: true keyword in favor of OpenAPI 3.1-style nullability (type: [x, "null"] or oneOf).
• Implemented the apply_required! method to populate the required array in the schema, handling both plain and grouped attributes while skipping those gated by a condition. [1] [2]
• Updated sanitized_documentation_hash and nested_definitions to conditionally remove the nullable: true keyword when the new option is enabled, preventing redundant or deprecated output. [1] [2]

To opt-in do this in you projects swagger_helper.rb (the default for these params is set to false)

LightweightSerializer::Documentation.new(klass).openapi_schema(
  emit_required: true,
  omit_deprecated_nullable: true
)