[ndc-graphql] union types are not supported causing crash running `ddn command add`

### Component

CLI -> c/v3-cli
Other connectors -> c/v3-ndc-graphql

### What is the current behaviour?

```log
ddn command add anilist "*"
1:38PM INF Using Subgraph config file "app/subgraph.yaml" found in context.
LOAD Subgraph at app/subgraph.yaml
ADD Command for source *, connector anilist in app
ADD Command for source Activity, connector anilist in app
ERROR Invalid ndc type ActivityUnion in connector anilist, subgraph app
MODIFIED (+8 more)
1:38PM ERR error adding command: exit status 2
```

### What is the expected behaviour?

No Error

### How to reproduce the issue?

1. Clone https://github.com/papaya-insurtech/mysupergraph
2. run `ddn command add anilist "*"`

### Screenshots or Screencast


### Please provide any traces or logs that could help here.

### Any possible solutions/workarounds you're aware of?

No

### Keywords

Start by reproducing `ddn command add anilist "*"` using the linked `mysupergraph` reproduction, then inspect the `c/v3-cli` and `c/v3-ndc-graphql` components involved in processing `ActivityUnion`. Done means the command completes without the reported invalid NDC type error when union types are present.

Written by the indexing model from the issue text.