[BUG] When attempting to run the action multiple times for the same PR, it fails after first time

### Error message

```
HTTP Error: 409, Channel `projects/PROJECT_NAME/sites/PROJECT_NAME/channels/pr47-main` already exists.
```

### Expected behavior

It should either:
a) Overwrite the PR's preview; or
b) Add some kind of random generated (maybe latest commit id?) string to the end (eg. `pr47-STRING_HERE-main` or something like that)

### Actual behavior

It crashes saying the site already exists.

Start by running the GitHub Action twice for the same pull request and capture the second-run failure. Trace how the action names the Firebase Hosting preview site or channel, then make repeated runs succeed using one of the expected behaviors and verify that the site is no longer rejected as already existing.

Written by the indexing model from the issue text.