fix(fetch): type narrowing with default response types (#443)

relates #439, and includes changes from #444 as the actual fix

- adds failing test showing that type narrowing isn't working as
expected for fetch clients when a default response is present
- adjusts the types to exclude specific status codes from the default
response branch. The specification states that the default response is
only to be used for responses not otherwise declared:
> The documentation of responses other than the ones declared for
specific HTTP response codes. Use this field to cover undeclared
responses.

---------

Co-authored-by: jeromeh <jerome.henaff@amdocs.com>

Author: mnahkies

e2e/openapi.yaml

@@ -277,6 +277,35 @@ paths:
       responses:
         500:
           description: Internal Server Error
+  /responses/default:
+    get:
+      tags:
+        - validation
+      parameters:
+        - name: status
+          in: query
+          schema:
+            type: string
+            enum: [200,500]
+      responses:
+        200:
+          description: ok
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  id:
+                    type: string
+        default:
+          description: Error
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  error:
+                    type: string
   /responses/empty:
     get:
       tags: