TLS configuration `cert_chain` field needs to specify that it needs to be the fullchain.pem, not chain.pem

If the TLS chain.pem is supplied, the Envoy proxy crashes with the error:

```
[2025-08-12 19:35:27.030][1][critical][main] [source/server/server.cc:416] error initializing config ' /envoy.json': Failed to load private key from /etc/certs/doltlab_key.pem, Cause: error:0b000074:X.509 certificate routines:OPENSSL_internal:KEY_VALUES_MISMATCH
```

Admins need to provide the fullchain.pem. This should be clear in the field names and in the documentation.