Changes

Improve adoption flow & message DTO handling

Refactor controllers and service to improve null-safety, validation and logging:

- AdoptionController: compacted DTO mapping to a single stream expression and removed unused import.
- MessageController: fixed method indentation, made toEntity null-safe, normalized isRead via Boolean.TRUE.equals(...), and ensured default timestamp handling.
- ShelterEventController: removed unused stream import.
- AdoptionService: added more descriptive logs, tightened pet availability check (includes pet ID in message), added IllegalOperationException to update/delete flows to prevent modifying/deleting FINISHED adoptions, added guards and warnings on delete, and minor formatting adjustments.
- AdopterServiceTest: removed an unused import.

These changes enforce business rules (cannot change/delete finished adoptions), improve robust handling of DTOs, and add clearer logging for troubleshooting.
 (commit: 74b4994)