S5: Testing & CI

[derpixler]

As a developer
I want automated tests and CI pipeline
So that the client is verified before release

Acceptance Criteria

• Unit tests for core classes (RequestHandler, DtoHydrator, ZammadClient, DTOs, Repositories)
• Integration test (Ticket CRUD lifecycle against live Zammad)
• CI pipeline with guard checks (unit job on push/PR, integration job on workflow_dispatch)
• PSR-12 compliance on all src/ files
• phpstan level=max, 0 errors
• phpcs PSR-12, 0 errors