5001: Add support for @Nullable annotations in OpenAPI 3.1 schemas

[zdary]

Pull Request

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Description

Enhanced schema generation by recognizing

• @Nullable annotations as nullable and adding "null" to the type array in OpenAPI 3.1.
• nullable = true in schema and adding "null" to the type array in OpenAPI 3.1.

Introduced a test case to validate the behavior and ensure backward compatibility with explicit @Schema annotations.

Fixes: #5001

Type of Change

• [X] 🐛 Bug fix
• [ ] ✨ New feature
• [ ] ♻️ Refactor (non-breaking change)
• [X] 🧪 Tests
• [ ] 📝 Documentation
• [ ] 🧹 Chore (build or tooling)

Checklist

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

Screenshots / Additional Context