test(transcription): mirror displayName length-cap regression on PUT endpoint

Sara #4 (PR #366 review). The 400-on-201-chars regression guard previously
only covered POST /api/documents/{id}/transcription-blocks. The same @Valid
cascade applies to PUT /api/documents/{id}/transcription-blocks/{blockId}
via UpdateTranscriptionBlockDTO, but no test asserted it — meaning a
silent removal of @Valid on the PUT @RequestBody parameter would slip past
CI. Mirror the test for symmetry.

Refs #362 #366

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

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

@@ -251,6 +251,21 @@ class TranscriptionBlockControllerTest {
                 .andExpect(jsonPath("$.label").value("Anrede"));
     }
 
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void updateBlock_returns400_whenMentionedPersonDisplayNameExceeds200Chars() throws Exception {
+        when(userService.findByEmail(any())).thenReturn(mockUser());
+        String longName = "A".repeat(201);
+        String body = "{\"text\":\"x\",\"mentionedPersons\":[{\"personId\":\""
+                + UUID.randomUUID() + "\",\"displayName\":\"" + longName + "\"}]}";
+
+        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 {