test(transcription): 400 + VALIDATION_ERROR when mention personId is null

Regression guard for the @NotNull on PersonMention.personId paired with
@Valid on the DTO field. The wiring was added in the previous commit; this
test ensures dropping either annotation in the future causes a loud test
failure rather than silently allowing payloads with no personId to reach
the service layer (where the listener relies on the UUID being present).

Refs #362

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -199,6 +199,20 @@ class TranscriptionBlockControllerTest {
                 .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
     }
 
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void createBlock_returns400_whenMentionedPersonPersonIdIsNull() throws Exception {
+        when(userService.findByEmail(any())).thenReturn(mockUser());
+        String body = "{\"pageNumber\":1,\"x\":0.1,\"y\":0.2,\"width\":0.3,\"height\":0.4,\"text\":\"x\","
+                + "\"mentionedPersons\":[{\"personId\":null,\"displayName\":\"Auguste Raddatz\"}]}";
+
+        mockMvc.perform(post(URL_BASE)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
+    }
+
     // ─── PUT /api/documents/{id}/transcription-blocks/{blockId} ─────────────
 
     @Test