contract: studio 계약의 VALIDATION_FAILED 개명(DOCUMENT_VALIDATION_FAILED) 반영

canonical studio-v1.yaml(tech-log-design-package b20d7a2)이 VALIDATION_FAILED를
DOCUMENT_VALIDATION_FAILED로 개명했다 — 스켈레톤 전역 OperationalError.
VALIDATION_FAILED(400)와 code 문자열이 충돌해 같은 code가 두 HTTP status를
갖던 문제를 해소한다.

- generate:tech-log-contract로 vendor된 계약·생성 타입·canonical-source.json 재생성
- STUDIO_ERROR_CODES(손수 유지되는 계약 미러)의 해당 항목 개명

계약 밖 코드는 STUDIO_UNAVAILABLE로 접히므로 이 배열이 계약과 어긋나면 안 된다.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/features/tech-log/contracts/studio/studio-api.openapi.yaml

@@ -611,7 +611,7 @@ components:
     RequestValidationFailed: { description: Request validation failed, x-error-codes: [REQUEST_VALIDATION_FAILED], content: { application/json: { schema: { $ref: "#/components/schemas/ErrorEnvelope" } } } }
     PreviewRejected:
       description: Preview 생성이 도메인 규칙으로 거절되었다
-      x-error-codes: [REQUEST_VALIDATION_FAILED, VALIDATION_FAILED, ASSET_NOT_READY, ASSET_QUARANTINED]
+      x-error-codes: [REQUEST_VALIDATION_FAILED, DOCUMENT_VALIDATION_FAILED, ASSET_NOT_READY, ASSET_QUARANTINED]
       content: { application/json: { schema: { $ref: "#/components/schemas/ErrorEnvelope" } } }
     PublishRejected:
       description: |
@@ -619,7 +619,7 @@ components:
 
         `WARNING_ACKNOWLEDGEMENT_REQUIRED`는 `acknowledgedWarningCodes`가
         현재 Validation의 WARNING 집합을 덮지 못한 경우다.
-      x-error-codes: [REQUEST_VALIDATION_FAILED, VALIDATION_FAILED, WARNING_ACKNOWLEDGEMENT_REQUIRED, ASSET_NOT_READY, ASSET_QUARANTINED]
+      x-error-codes: [REQUEST_VALIDATION_FAILED, DOCUMENT_VALIDATION_FAILED, WARNING_ACKNOWLEDGEMENT_REQUIRED, ASSET_NOT_READY, ASSET_QUARANTINED]
       content: { application/json: { schema: { $ref: "#/components/schemas/ErrorEnvelope" } } }
     AssetRejected:
       description: Asset metadata 변경이 거절되었다
@@ -650,7 +650,7 @@ components:
         code:
           type: string
           enum: [AUTHENTICATION_REQUIRED, STUDIO_ACCESS_DENIED, DOCUMENT_NOT_FOUND, VERSION_CONFLICT,
-                 REQUEST_VALIDATION_FAILED, VALIDATION_FAILED, VALIDATION_STALE, PREVIEW_NOT_FOUND,
+                 REQUEST_VALIDATION_FAILED, DOCUMENT_VALIDATION_FAILED, VALIDATION_STALE, PREVIEW_NOT_FOUND,
                  PREVIEW_STALE, PREVIEW_EXPIRED, PUBLICATION_NOT_FOUND, PUBLICATION_CONFLICT,
                  PUBLICATION_EVENT_NOT_FOUND, PUBLICATION_SNAPSHOT_NOT_FOUND,
                  WARNING_ACKNOWLEDGEMENT_REQUIRED, IDEMPOTENCY_KEY_REUSED, ASSET_NOT_FOUND,