[CLAUDE] PurchaseEvaluation: Chunk A — BE guard target TraLai/TuChoi BẮT BUỘC decision=Reject + 3 regression test

Defense-in-depth chặn FE inconsistency (gotcha #45 — Session 21 turn 3).
Bug pattern: button "← Trả lại" trong PeWorkflowPanel.tsx gửi decision=Approve
khi target=TraLai do `isReject` local var thiếu nhánh TraLai → BE skip Reject
branch → enter APPROVE STEP → ApproveV2Async UPSERT opinion = "đã duyệt" +
advance Cấp. User UAT thấy: "Trả về nhưng hệ thống vẫn duyệt".

BE guard:
- Service `TransitionAsync` thêm early check sau set isAdmin/isSystem
- targetPhase ∈ {TraLai, TuChoi} && decision != Reject → throw ConflictException
- Boundary protection cho mọi caller tương lai (API client / mobile / cron)

Tests (Infra suite +3):
- TransitionAsync_TargetTraLai_WithApproveDecision_Throws_AndDoesNotMutateState
- TransitionAsync_TargetTuChoi_WithApproveDecision_Throws_AndDoesNotMutateState
- TransitionAsync_TargetTraLai_WithRejectDecision_SetsPhaseTraLai (happy path)
+ NoOpNotificationService stub reusable cho future PE service tests

Verify:
- dotnet test SolutionErp.slnx → 84 PASS (58 Domain + 26 Infra = +3 from 81 baseline)
- Build pass (0 err, 2 warn CS8602 pre-existing DocxRenderer)

Pending Chunk B: FE fix PeWorkflowPanel.tsx isReject + dialog isSendBack
mirror 2 app (fe-admin + fe-user) — sync với BE guard rule.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Backend/SolutionErp.Infrastructure/Services/PurchaseEvaluationWorkflowService.cs

@@ -47,6 +47,26 @@ public class PurchaseEvaluationWorkflowService(
         var isAdmin = actorRoles.Contains(AppRoles.Admin);
         var isSystem = actorUserId is null && decision == ApprovalDecision.AutoApprove;
 
+        // ===== GUARD: targetPhase TraLai/TuChoi BẮT BUỘC decision=Reject =====
+        // Defense-in-depth chặn FE inconsistency (gotcha #45 — Session 21 turn 3):
+        //   Bug: button "← Trả lại" trong PeWorkflowPanel.tsx gửi decision=Approve
+        //   khi target=TraLai do `isReject` local var thiếu nhánh TraLai. BE nhận
+        //   payload sẽ skip Reject branch → enter APPROVE STEP → ApproveV2Async
+        //   UPSERT opinion = "đã duyệt" + advance Cấp. User UAT thấy: "Trả về
+        //   nhưng hệ thống vẫn duyệt".
+        // FE fix song song trong fe-admin + fe-user (rule §3.9 mirror 2 app).
+        // Guard này KHÔNG xoá khi FE fix — boundary protection cho mọi caller
+        // tương lai (API client / mobile app / cron retry).
+        if ((targetPhase == PurchaseEvaluationPhase.TraLai
+                || targetPhase == PurchaseEvaluationPhase.TuChoi)
+            && decision != ApprovalDecision.Reject)
+        {
+            throw new ConflictException(
+                $"Transition tới {targetPhase} BẮT BUỘC decision=Reject (nhận {decision}). " +
+                "Báo lỗi caller — payload mismatch giữa target phase và decision " +
+                "(xem gotcha #45 + docs/workflow-contract.md).");
+        }
+
         // ===== REJECT BRANCH =====
         if (decision == ApprovalDecision.Reject)
         {