Server-side Piet protocol validator.

This checks for nearly all the server-side conditions in errors.proto, including missing/duplicate styles (respecting MediaQueryCondition), missing/duplicate/mismatched BindingValues, missing content in Elements, etc.

PiperOrigin-RevId: 277795802
Change-Id: I88bde10dbe068bec89b186097c262a89cf849689
diff --git a/src/main/proto/search/now/ui/piet/errors.proto b/src/main/proto/search/now/ui/piet/errors.proto
index f53d1ab..ec43a36 100644
--- a/src/main/proto/search/now/ui/piet/errors.proto
+++ b/src/main/proto/search/now/ui/piet/errors.proto
@@ -224,6 +224,14 @@
     (client_error) = ERROR
   ] */;
 
+  // Bindings are not supported in Frames or within Bound Elements.
+  ERR_UNSUPPORTED_CONTEXT_FOR_BINDING = 11 /* [
+    // Bindings will not work in this context
+    (server_error) = ERROR,
+    // Clients might support bindings in this context.
+    (client_error) = WARNING
+  ] */;
+
   // ---------------------------------------------------------------------------
   // Missing required proto fields, or invalid values.
   // Fields start at ID 101.