Mark channel credentials channel arg as internal (#27752)

* Mark channel credentials channel arg as internal

* add reproducer

src/core/lib/security/credentials/credentials.h

@@ -93,7 +93,7 @@ void grpc_override_well_known_credentials_path_getter(
 
 /* --- grpc_channel_credentials. --- */
 
-#define GRPC_ARG_CHANNEL_CREDENTIALS "grpc.channel_credentials"
+#define GRPC_ARG_CHANNEL_CREDENTIALS "grpc.internal.channel_credentials"
 
 // This type is forward declared as a C struct and we cannot define it as a
 // class. Otherwise, compiler will complain about type mismatch due to

test/core/end2end/fuzzers/api_fuzzer_corpus/testcase-5212802075197440

@@ -0,0 +1,17 @@
+actions {
+  create_channel {
+    target: "unix::\360,\206::::::::::::\354\327::::::::::::::::::\213\300:::::174515:::::::::::::::::, \310/:::::::::::::::::::::::::::::::::::::::?:::::::::::::::::::::::::;::::::::::::::"
+    channel_args {
+      key: "grpc.channel_credentials"
+      resource_quota {
+      }
+    }
+    channel_creds {
+      ssl_channel_creds {
+      }
+    }
+  }
+}
+actions {
+  check_connectivity: true
+}