Remove panic in type mappings, allow arrays in command return types (#1425)

<!-- The PR description should answer 2 important questions: -->

### What

When an array return type was used in a command that we using in type
mappings we panicked, however this should be allowed. Return a proper
error if a Predicate is used as a command return type though, as that is
not allowed.

This fixes the immediate error in the ticket, but the reproducer throws
another one - I think this is because the reproduction is so minimal
though. Worth testing with the full reproduction before merging
@jberryman .

V3_GIT_ORIGIN_REV_ID: ae0a2f0a6fe83c82e5ab01adaba9b317743688ba

v3/changelog.md

@@ -6,6 +6,9 @@
 
 ### Fixed
 
+- Fixed a bug where commands with array return types would not build when header
+  forwarding was in effect.
+
 - GraphQL queries with `order_by` arguments that contain multiple properties set
   on one input object now properly return an error. For example
   `order_by: { location: { city: Asc, country: Asc } }` is no longer allowed.

v3/crates/metadata-resolve/src/helpers/type_mappings.rs

@@ -38,6 +38,10 @@ pub enum TypeMappingCollectionError {
         data_connector: Qualified<DataConnectorName>,
         ndc_type_name: DataConnectorObjectType,
     },
+
+    #[error("Cannot return a predicate type from a command")]
+    PredicateAsResponseType,
+
     #[error("Internal Error: Unknown type {type_name:} when collecting type mappings")]
     InternalUnknownType {
         type_name: Qualified<CustomTypeName>,
@@ -176,7 +180,10 @@ fn handle_special_case_type_mapping<'a>(
                 .get(response_config.result_field.as_str())
                 .unwrap()
                 .r#type;
-            let ndc_object_type_name = unwrap_ndc_object_type_name(ndc_object_type);
+
+            // get the type found in `response` field, and look up it's type mappings too
+            let ndc_object_type_name = unwrap_ndc_object_type_name(ndc_object_type)?;
+
             object_type_representation
                 .type_mappings
                 .get(data_connector_name, ndc_object_type_name.as_str())
@@ -207,14 +214,17 @@ fn handle_special_case_type_mapping<'a>(
     }
 }
 
-fn unwrap_ndc_object_type_name(ndc_type: &ndc_models::Type) -> &ndc_models::TypeName {
+fn unwrap_ndc_object_type_name(
+    ndc_type: &ndc_models::Type,
+) -> Result<&ndc_models::TypeName, TypeMappingCollectionError> {
     match ndc_type {
-        ndc_models::Type::Named { name } => name,
+        ndc_models::Type::Named { name } => Ok(name),
         ndc_models::Type::Nullable { underlying_type } => {
             unwrap_ndc_object_type_name(underlying_type)
         }
-        ndc_models::Type::Array { .. } | ndc_models::Type::Predicate { .. } => {
-            panic!("unexpected ndc type; only object is supported")
+        ndc_models::Type::Array { element_type } => unwrap_ndc_object_type_name(element_type),
+        ndc_models::Type::Predicate { .. } => {
+            Err(TypeMappingCollectionError::PredicateAsResponseType)
         }
     }
 }