test(transcription): add updateBlock 400 test for null personId in mention

createBlock has both validation guards (displayName length + personId null).
updateBlock had only the displayName test. Add the symmetric null-personId case
so a future @Valid drop from updateBlock's @RequestBody would be caught.

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

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

@@ -266,6 +266,19 @@ class TranscriptionBlockControllerTest {
                 .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
     }
 
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void updateBlock_returns400_whenMentionedPersonPersonIdIsNull() throws Exception {
+        when(userService.findByEmail(any())).thenReturn(mockUser());
+        String body = "{\"text\":\"x\",\"mentionedPersons\":[{\"personId\":null,\"displayName\":\"Auguste Raddatz\"}]}";
+
+        mockMvc.perform(put(URL_BLOCK)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
+    }
+
     @Test
     @WithMockUser(authorities = "WRITE_ALL")
     void updateBlock_returns404_whenBlockDoesNotExist() throws Exception {