fix: OAS 3.1 schema generation for raw Object properties

[xeulbn]

Pull Request

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Description

Related issue: #4682
This PR fixes an issue in OpenAPI 3.1 schema generation where properties
declared as raw Java Object are rendered as an empty schema ({}),
instead of an explicit object schema.

The issue does not occur in OAS 3.0.x, but appears in OAS 3.1 due to the
JSON Schema–based handling of types, where Schema#getTypes() is used
instead of Schema#getType() during serialization.

To address this, the OAS 3.1 schema generation for raw Object types
has been updated to explicitly set types = ["object"].

Additionally, regression tests have been added to cover:

• raw Object properties
• List<Object> item schemas
• Map<String, Object> additionalProperties schemas

Fixes: #4682

Type of Change

• 🐛 Bug fix
• 🧪 Tests

Checklist

• I have added/updated tests as needed
• The PR title is descriptive
• The code builds and passes tests locally
• I have linked related issues (if any)

Screenshots / Additional Context

The tests were executed locally with:

mvn -pl modules/swagger-core -Dtest=ObjectFieldOas31ReproTest test

Before (OAS 3.1)

After (OAS 3.1)