Proxy Defaults - Enabling Transparent Proxy Impacts API Gateway Service Registration

#### Overview of the Issue

Setting `mode = "transparent"` in the proxy-defaults configuration entry causes subsequent API Gateway service registration to fail with -

```log
2023-10-31T20:20:35.192Z [ERROR] agent.client: RPC failed to server: method=Catalog.Register server=172.24.0.8:8300
error=
| rpc error making call: 1 error occurred:
| \t* The Proxy.Upstreams configuration is invalid for a api-gateway
|

2023-10-31T20:20:35.192Z [WARN] agent: Syncing service failed.: service=default/default/api-gateway
error=
| rpc error making call: 1 error occurred:
| \t* The Proxy.Upstreams configuration is invalid for a api-gateway
|

2023-10-31T20:20:35.192Z [ERROR] agent: failed to sync changes:
error=
| 1 error occurred:
| \t* rpc error making call: 1 error occurred:
| \t* The Proxy.Upstreams configuration is invalid for a api-gateway
|
|
|

Registered service: api-gateway
```

> Note: Although the client agent log output suggests the `api-gateway` service was registered after the error, the service is never synchronized to the catalog.

For context, I had enabled the dependencies for Permissive mTLS and was deploying API Gateway to the same partition when this issue was uncovered. Applying transparent proxy on individual services/service-defaults allows API Gateway service registration to succeed.

---

#### Reproduction Steps

```sh
curl "${CONSUL_HTTP_ADDR}/v1/config" \
--header "X-Consul-Token: $CONSUL_HTTP_TOKEN" \
--request PUT \
--data @- <

- VM-based
- Consul Ent 1.16.2

Start by reproducing the proxy-defaults configuration and API Gateway registration sequence in the issue, then inspect the proxy-defaults handling and API Gateway service-registration entry points implicated by the error. Confirm the fix by repeating the commands and verifying that api-gateway is synchronized to the catalog without the invalid Proxy.Upstreams error.

Written by the indexing model from the issue text.