test(grpc): use grpc-test-old model and track it for CI

grpc-test.json was replaced with a different model; tests expect
SayHello1/SayHello2 and link-target structure. Use grpc-test-old.json
and add it to repo (exception in .gitignore) so CI passes.

Made-with: Cursor

Author: alexpmule

.gitignore

@@ -64,6 +64,7 @@ dist/
 !demo/avro.json
 !demo/avro2.json
 !demo/grpc-test.json
+!demo/grpc-test-old.json
 !demo/product-order-minimal.json
 !demo/product-order-minimal-compact.json
 !demo/product-order-deep-allof.json