S5: Unit Tests

[derpixler]

Add PHPUnit unit tests for:

• RequestHandler (exception mapping — 200, 401, 404, 422, 429, 503, network error)
• DtoHydrator (DTO hydration from arrays)
• ZammadClient (repository memoization, invalid repo error)
• TicketDTO (fromArray/toArray roundtrip)
• TicketRepository (pagination generator — 2 pages)
• TicketArticleRepository (repository wiring)

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Parent: #146 S6: Testing & CI