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>