[Flight] Don't try to close debug channel twice (#34340)

When the debug channel was already closed, we must not try to close it
again when the Response gets garbage collected.

**Test plan:**

1. reduce the Flight fixture `App` component to a minimum [^1]
    - remove everything from `<body>`
    - delete the `console.log` statement
2. open the app in Firefox (seems to have a more aggressive GC strategy)
3. wait a few seconds

On `main`, you will see the following error in the browser console:

```
TypeError: Can not close stream after closing or error
```

With this change, the error is gone.

[^1]: It's a bit concerning that step 1 is needed to reproduce the
issue. Either GC is behaving differently with the unmodified App, or we
may hold on to the Response under certain conditions, potentially
creating a memory leak. This needs further investigation.

Hendrik Liebau committed 8mo ago

aad7c664ffbde52e5d8004b542d83d6d4b7a32a0

Parent: 3fe51c9

Committed by GitHub <noreply@github.com> on 8/29/2025, 3:22:39 PM