[TASK] Migrate Persona Key Tag Field to new Custom Field API

[erickgonzalez]

Description

We need to migrate the Content Type Persona – Key Tag custom field from the empty starter implementation to the new Custom Field API.

The migration must ensure the custom field works correctly in New Edit Mode (Recommended).
Support for New Edit Mode (Deprecated) is explicitly out of scope.

As part of this effort, we should continue using the existing easy switch mechanism between code versions, allowing the appropriate implementation to run depending on the active Edit Content phase.

This migration should also capture and document any relevant learnings that can be reused for future custom field migrations and the upcoming Go-To-Market phase.

Acceptance Criteria

• Persona Key Tag custom field is fully migrated to the new Custom Field API
• Custom field works correctly in New Edit Mode (Recommended)
• Deprecated Edit Mode is not required to work and is not supported
• Existing mechanism to switch between code versions is preserved
• No regressions in Persona behavior or tag handling
• Migration learnings are documented for future reference

Priority

Medium

Additional Context

No response