feat(sidekick/swift): use rpc() helper (#6217)

Use the `GoogleCloudGax.HTTPClient.rpc()` as it captures service errors
with full details. Arguably the generated code is a little shorter too,
which is not a bad thing.

Author: coryan

internal/sidekick/swift/templates/common/stub.swift.mustache

@@ -82,18 +82,14 @@ extension Clients {
       }
       {{/Codec.IsBodyWildcard}}
       {{/Codec.HasBody}}
-      let (data, response) = try await self.inner.data(for: req)
-      if !(200..<300).contains(response.statusCode) {
-        throw GoogleCloudGax.RequestError.http(GoogleCloudGax.HTTPDetails(
-          http_status_code: response.statusCode,
-          headers: [:],
-          payload: data,
-        ))
-      }
       {{^ReturnsEmpty}}
+      let (data, _) = try await self.inner.rpc(for: req).get()
       return try {{InputType.Codec.Model.WktPackage}}._ProtoJSONDecoder().decode(
         {{Codec.ReturnType}}.self, from: data)
       {{/ReturnsEmpty}}
+      {{#ReturnsEmpty}}
+      _ = try await self.inner.rpc(for: req).get()
+      {{/ReturnsEmpty}}
     }
     {{/Codec.RestMethods}}
   }