fix(population-density-data): address validation warnings and hints from Sync26 - #149

[S-035] Notification callback content type must include 'application/cloudevents+json', found: application/json

[P-015] No event type enum values found in code/API_definitions/population-density-data.yaml — subscription APIs should define EventType schemas | Suggestion: Define a named event type schema (e.g. ApiEventType) that constrains the CloudEvent `type` value via allOf, rather than inlining the enum directly in CloudEvent.properties.type.enum. See the implicit-events API template in Commonalities artifacts/api-templates/ (tracked in camaraproject/Commonalities#608).

[S-313] Schema of type string should specify a format, pattern, enum, or const. | Suggestion: Acceptable if free-form field or implementation-dependent — no fix needed.