feat: Add reviewers field to PUT /merge-request/{id}

- New MergeRequestReviewerRepository with diff-based setReviewers()
  and eligibility validation (delegates to AdminProjectRepository::getReviewers)
- New InvalidReviewerException with string code storage.mergeRequests.invalidReviewer
- MergeRequestUpdateRequest: optional reviewers field (PATCH semantics — omit to keep)
- MergeRequestUpdateProcessor: calls setReviewers when field is present
- MergeRequestUpdateAction: catches InvalidReviewerException → HTTP 422
- MergeRequestResponseProvider: includes reviewers array in all MR responses
- MergeRequestResponse: OA schema updated
- E2E tests: happy-path (add/omit/clear) + invalid reviewer ID

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: martinsifra

src/Keboola/StorageApi/Client.php

@@ -1019,7 +1019,8 @@ public function getMergeRequest(int $mergeRequestId): array
      *     description?: ?string,
      *     externalId?: ?string,
      *     autoMergeStrategy?: ?string,
-     *     autoMergeAt?: ?string
+     *     autoMergeAt?: ?string,
+     *     reviewers?: list<int>
      * } $data
      * @return array mrData
      */