fix(population-density-data): address validation warnings and hints from Sync26

[albertoramosmonagas]

What type of PR is this?

Add one of the following kinds:

• correction

What this PR does / why we need it:

Addresses deferred CAMARA validation findings from the Commonalities r4.4 synchronization. This PR resolves 10 warnings and 4 hints to achieve clean validation status:

• Updates notification callback media type to application/cloudevents+json
• Adds descriptions for array items and schema properties
• Updates apiRoot description to match standard CAMARA wording
• Implements validation hints for OpenAPI compliance
• Fixes Gherkin indentation in test definitions

Which issue(s) this PR fixes:

Fixes #130

Special notes for reviewers:

• Validation findings P-015, S-035, S-313 intentionally not addressed (not applicable)
• All other findings have been resolved
• See validation run: https://github.com/camaraproject/PopulationDensityData/actions/runs/35337063885

Changelog input

 release-note
Resolved CAMARA validation findings for Commonalities r4.4 synchronization

[jgarciahospital]

LGTM