Don't make oneOf errors tagged unions

OpenAPI only lets you have one error per HTTP response code. This
is a problem when converting things because Smithy has no such
limitation. To address that, we added a toggle that lets you
disambiguate by making the body contents a `oneOf`, which is an
untagged union. The problem is that we made this a tagged union by
leaning on how Smithy converts Smithy's union shape, which is tagged.

To illustrate the problem, the OpenAPI modeled body technically now
declares the following body valid:

```json
{
    "Error1" {
        "foo": "bar"
    }
}
```

But the model for the structure is:

```
@error("client")
@HttpError(400)
structure Error1 {
    foo: String
}
```

And an actual valid body would be:

```json
{
    "foo": "bar"
}
```

This commit turns the OpenAPI body into an untagged union instead,
which is how it should be.

Author: JordonPhillips

smithy-openapi/src/main/java/software/amazon/smithy/openapi/fromsmithy/protocols/AwsRestJson1Protocol.java

@@ -6,6 +6,7 @@
 
 import static software.amazon.smithy.openapi.OpenApiConfig.ErrorStatusConflictHandlingStrategy.ONE_OF;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Set;
 import java.util.TreeSet;
@@ -130,16 +131,15 @@ Schema createDocumentSchema(
         if (context.getConfig().getOnErrorStatusConflict() != null
                 && context.getConfig().getOnErrorStatusConflict().equals(ONE_OF)
                 && targetsSyntheticError(cleanedShape, context)) {
-            UnionShape.Builder asUnion = UnionShape.builder().id(cleanedShape.getId());
             UnionShape targetUnion = context.getModel()
                     .expectShape(
                             cleanedShape.getAllMembers().values().stream().findFirst().get().getTarget(),
                             UnionShape.class);
+            List<Schema> oneOfMembers = new ArrayList<>();
             for (MemberShape member : targetUnion.getAllMembers().values()) {
-                String name = member.getMemberName();
-                asUnion.addMember(member.toBuilder().id(cleanedShape.getId().withMember(name)).build());
+                oneOfMembers.add(context.createRef(member));
             }
-            return context.getJsonSchemaConverter().convertShape(asUnion.build()).getRootSchema();
+            return Schema.builder().type(null).oneOf(oneOfMembers).build();
         }
         return context.getJsonSchemaConverter().convertShape(cleanedShape).getRootSchema();
     }

smithy-openapi/src/test/resources/software/amazon/smithy/openapi/fromsmithy/protocols/error-code-collision-test-use-oneof.openapi.json

@@ -154,104 +154,32 @@
       "GetCurrentTime404ErrorResponseContent": {
         "oneOf": [
           {
-            "type": "object",
-            "title": "Error7",
-            "properties": {
-              "Error7": {
-                "$ref": "#/components/schemas/Error7"
-              }
-            },
-            "required": [
-              "Error7"
-            ]
+            "$ref": "#/components/schemas/Error7"
           },
           {
-            "type": "object",
-            "title": "Error8",
-            "properties": {
-              "Error8": {
-                "$ref": "#/components/schemas/Error8"
-              }
-            },
-            "required": [
-              "Error8"
-            ]
+            "$ref": "#/components/schemas/Error8"
           }
         ]
       },
       "GetCurrentTime429ErrorResponseContent": {
         "oneOf": [
           {
-            "type": "object",
-            "title": "Error1",
-            "properties": {
-              "Error1": {
-                "$ref": "#/components/schemas/Error1"
-              }
-            },
-            "required": [
-              "Error1"
-            ]
+            "$ref": "#/components/schemas/Error1"
           },
           {
-            "type": "object",
-            "title": "Error2",
-            "properties": {
-              "Error2": {
-                "$ref": "#/components/schemas/Error2"
-              }
-            },
-            "required": [
-              "Error2"
-            ]
+            "$ref": "#/components/schemas/Error2"
           },
           {
-            "type": "object",
-            "title": "Error3",
-            "properties": {
-              "Error3": {
-                "$ref": "#/components/schemas/Error3"
-              }
-            },
-            "required": [
-              "Error3"
-            ]
+            "$ref": "#/components/schemas/Error3"
           },
           {
-            "type": "object",
-            "title": "Error4",
-            "properties": {
-              "Error4": {
-                "$ref": "#/components/schemas/Error4"
-              }
-            },
-            "required": [
-              "Error4"
-            ]
+            "$ref": "#/components/schemas/Error4"
           },
           {
-            "type": "object",
-            "title": "Error5",
-            "properties": {
-              "Error5": {
-                "$ref": "#/components/schemas/Error5"
-              }
-            },
-            "required": [
-              "Error5"
-            ]
+            "$ref": "#/components/schemas/Error5"
           },
           {
-            "type": "object",
-            "title": "Error6",
-            "properties": {
-              "Error6": {
-                "$ref": "#/components/schemas/Error6"
-              }
-            },
-            "required": [
-              "Error6"
-            ]
+            "$ref": "#/components/schemas/Error6"
           }
         ]
       },
@@ -269,4 +197,4 @@
       }
     }
   }
-}
+}