Fix YAML syntax error reporting on non-map YAML (e.g. "hello")

PiperOrigin-RevId: 889328156

Author: dmitriplotnikov

env/env_yaml.cc

@@ -998,6 +998,15 @@ void EmitFunctionConfigs(const Config& env_config, YAML::Emitter& out) {
 absl::StatusOr<Config> EnvConfigFromYaml(const std::string& yaml) {
   Config config;
   CEL_ASSIGN_OR_RETURN(YAML::Node root, LoadYaml(yaml));
+  if (!root.IsDefined() || root.IsNull()) {
+    return config;
+  }
+
+  if (!root.IsMap()) {
+    return absl::InvalidArgumentError(FormatYamlErrorMessage(
+        yaml, "Invalid CEL environment config YAML", root.Mark()));
+  }
+
   CEL_RETURN_IF_ERROR(ParseName(config, yaml, root));
   CEL_RETURN_IF_ERROR(ParseContainerConfig(config, yaml, root));
   CEL_RETURN_IF_ERROR(ParseExtensionConfigs(config, yaml, root));

env/env_yaml_test.cc

@@ -528,6 +528,12 @@ TEST_P(EnvYamlParseTest, EnvYamlSyntaxError) {
 INSTANTIATE_TEST_SUITE_P(
     EnvYamlParseTest, EnvYamlParseTest,
     ::testing::Values(
+        ParseTestCase{
+            .yaml = R"yaml( invalid yaml )yaml",
+            .expected_error = "1:2: Invalid CEL environment config YAML\n"
+                              "| invalid yaml \n"
+                              "| ^",
+        },
         ParseTestCase{
             .yaml = R"yaml(
                 name: