Expr: Make math expression scalars compliant with dataplane contract (#90350)

* Specify frame type for scalars arbitrarily

* Update alerting tests which assert on entire dataframe fields

pkg/expr/mathexp/types.go

@@ -88,7 +88,10 @@ func (s Scalar) AsDataFrame() *data.Frame { return s.Frame }
 func NewScalar(name string, f *float64) Scalar {
 	frame := data.NewFrame("",
 		data.NewField(name, nil, []*float64{f}),
-	)
+	).SetMeta(&data.FrameMeta{
+		Type:        data.FrameTypeNumericMulti,
+		TypeVersion: data.FrameTypeVersion{0, 1},
+	})
 	return Scalar{frame}
 }