eliminate legacy replace path and unify to operator-based execution

- Route ODKU all-column-self-update rewrite through modern path
- Support fake PK tables (no PK + UK) in modern path
- Support vector index tables in modern path (skip irregular indexes)
- Remove legacy build_replace.go, scope.replace(), and all related code
- Clean up proto: mark ReplaceCtx, Node.REPLACE, Query.REPLACE as reserved
- Remove Node_REPLACE references from explain code
- Add BVT tests for NULL UK, fake PK+UK, and ODKU scenarios

Issue: #23946

Author: ck89119

pkg/frontend/mysql_cmd_executor.go

@@ -2302,12 +2302,6 @@ func checkModify(plan0 *plan.Plan, resolveFn func(string, string, *plan2.Snapsho
 					return true, err
 				}
 			}
-			if ctx := p.Query.Nodes[i].ReplaceCtx; ctx != nil {
-				flag, err := checkFn(ctx.Ref, ctx.TableDef)
-				if err != nil || flag {
-					return true, err
-				}
-			}
 			if ctx := p.Query.Nodes[i].DeleteCtx; ctx != nil {
 				flag, err := checkFn(ctx.Ref, ctx.TableDef)
 				if err != nil || flag {

pkg/frontend/mysql_cmd_executor_test.go

@@ -2445,13 +2445,6 @@ func Test_checkModify(t *testing.T) {
 			expected_flag: true,
 			expected_err:  false,
 		},
-		{
-			node: &plan.Node{
-				ReplaceCtx: &plan0.ReplaceCtx{},
-			},
-			expected_flag: true,
-			expected_err:  false,
-		},
 		{
 			node: &plan.Node{
 				DeleteCtx: &plan0.DeleteCtx{},