feat: add a test and fail assignment expressions

Signed-off-by: Manan Gupta <manan@planetscale.com>

go/vt/sqlparser/normalizer.go

@@ -157,7 +157,10 @@ func newNormalizer(
 // It handles normalization logic based on node types.
 func (nz *normalizer) walkDown(node, _ SQLNode) bool {
 	switch node := node.(type) {
-	case *Begin, *Commit, *Rollback, *Savepoint, *SRollback, *Release, *OtherAdmin, *Analyze, *AssignmentExpr,
+	case *AssignmentExpr:
+		nz.err = vterrors.VT12001("Assignment expression")
+		return false
+	case *Begin, *Commit, *Rollback, *Savepoint, *SRollback, *Release, *OtherAdmin, *Analyze,
 		*PrepareStmt, *ExecuteStmt, *FramePoint, *ColName, TableName, *ConvertType:
 		// These statement don't need normalizing
 		return false

go/vt/vtgate/planbuilder/testdata/unsupported_cases.json

@@ -229,6 +229,11 @@
     "query": "select @val := 42",
     "plan": "VT12001: unsupported: Assignment expression"
   },
+  {
+    "comment": "Assignment expression in on duplicate clause",
+    "query": "insert into unsharded (id) values (@val := 42)",
+    "plan": "VT12001: unsupported: Assignment expression"
+  },
   {
     "comment": "Assignment expression in union statements",
     "query": "select @val := 42 union select 1",