fix(sample-gen): should return xml literal example

[mathis-m]

Description

A string example for an XML schema should be returned as provided.
In sampleFromSchemaGeneric added in case xml object schema the conditional return for the string example.
Added in createXMLExample the conditional return for the string example.

Motivation and Context

Fixes #6818

How Has This Been Tested?

added unit test to ensure literal example is returned as is.

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 can you fix the merge conflict in this PR? Looks like I am merging out of sequence. =)

[mathis-m]

@tim-lai I accidently added tests on exact same lines and they don't differ two much in two PRs, that is why merge is struggling.

[tim-lai]

no problem, thanks for the update!