Fix/array item schema

[mathis-m]

Description

Fixes wrong array items schema selection logic, for the schema which will passed to jsonschema components.

items.schema is not defined in oas. Instead the schema is directly under items.

I also enhanced the ui by adding type text to add button:

Motivation and Context

Fixes #6326

How Has This Been Tested?

Not

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[tim-lai]

@mathis-m PR merged! Thanks for the fix. Fyi, the feat(ux) would have been better as a separate PR, but I noticed it too late. ;)