all: statically enforce json/v2 interface satisfaction (#15154)

The json/v2 prototype is still in flux and the API can/will change.

Statically enforce that types implementing the v2 methods
satisfy the correct interface so that changes to the signature
can be statically detected by the compiler.

Updates tailscale/corp#791

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/syspolicy/setting/raw_item.go

@@ -75,6 +75,11 @@ func (i RawItem) String() string {
 	return fmt.Sprintf("%v%s", i.data.Value.Value, suffix)
 }
 
+var (
+	_ jsonv2.MarshalerTo     = (*RawItem)(nil)
+	_ jsonv2.UnmarshalerFrom = (*RawItem)(nil)
+)
+
 // MarshalJSONTo implements [jsonv2.MarshalerTo].
 func (i RawItem) MarshalJSONTo(out *jsontext.Encoder) error {
 	return jsonv2.MarshalEncode(out, &i.data)
@@ -114,6 +119,11 @@ func RawValueOf[T RawValueType](v T) RawValue {
 	return RawValue{opt.ValueOf[any](v)}
 }
 
+var (
+	_ jsonv2.MarshalerTo     = (*RawValue)(nil)
+	_ jsonv2.UnmarshalerFrom = (*RawValue)(nil)
+)
+
 // MarshalJSONTo implements [jsonv2.MarshalerTo].
 func (v RawValue) MarshalJSONTo(out *jsontext.Encoder) error {
 	return jsonv2.MarshalEncode(out, v.Value)