Enrollment handing for containerized agents

For Fleet-managed Elastic Agents that run in a containerized environment (including Docker, Kubernetes, and others), enrollment handling is managed as follows:

• Enrollment Verification: Elastic Agent checks the stored enrollment conditions within its container environment and re-enrolls only when necessary.
• Unenrollment Handling: If an Elastic Agent is unenrolled via the Fleet UI but still references a valid enrollment token provided through environment variables, it will automatically re-enroll on the next container restart.

In versions lower than 8.18, an unenrolled agent remains unenrolled and does not re-enroll, even if a valid enrollment token is still available.