test(persons): assert error code in createPerson_returns400_whenPersonTypeIsSkip

Adds jsonPath("$.code").value("INVALID_PERSON_TYPE") to verify the full
error response shape, not just the HTTP status.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/controller/PersonControllerTest.java

@@ -272,7 +272,8 @@ class PersonControllerTest {
         mockMvc.perform(post("/api/persons")
                         .contentType(MediaType.APPLICATION_JSON)
                         .content("{\"lastName\":\"Müller\",\"personType\":\"SKIP\"}"))
-                .andExpect(status().isBadRequest());
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("INVALID_PERSON_TYPE"));
     }
 
     // ─── PUT /api/persons/{id} ────────────────────────────────────────────────