[BUG]: github_repository_environment fails to refresh when an Enterprise Team is a required reviewer (reviewer.Type "BusinessTeam")

### Expected Behavior

Reading/refreshing a `github_repository_environment` whose required reviewers include a GitHub **Enterprise Team** should succeed — either representing that reviewer in state, or ignoring it gracefully.

### Actual Behavior

Any `plan`/`apply` that refreshes the resource fails:

```
Error: reviewer.Type is *string, not a string of 'User' or 'Team', unable to unmarshal

with github_repository_environment.notify_gate[""],
on main.repos.tf line 232, in resource "github_repository_environment" "notify_gate":
232: resource "github_repository_environment" "notify_gate" {
```

Because Terraform refreshes the whole state, **a single environment with an Enterprise Team reviewer blocks every `plan`/`apply` in the workspace**, including for completely unrelated resources. The resource becomes unmanageable until the Enterprise Team reviewer is removed out-of-band via the REST API or UI.

### Terraform Version

```
Terraform v1.15.8
on windows_amd64
+ provider registry.terraform.io/integrations/github v6.13.0
```

### GitHub Installation Type

- [ ] GitHub.com (Free, Pro, or Team)
- [ ] GitHub Enterprise Server (on-premises)
- [ ] GitHub Enterprise Cloud with Personal Accounts (github.com)
- [x] GitHub Enterprise Cloud with Managed Users/EMU (github.com)
- [ ] GitHub Enterprise Cloud with Data Residency (*.ghe.com)
- [ ] I don't know

### Affected Resource(s)

- `github_repository_environment` (read/refresh path)
- Likely anything else decoding required reviewers from the environments API (e.g. the corresponding data source)

### Terraform Configuration Files

```hcl
resource "github_repository_environment" "notify_gate" {
for_each = var.repositories
environment = "notify-gate"
repository = github_repository.iac[each.key].name

reviewers {
users = [2363016, 1317356] # two named users
}
}

The Enterprise Team reviewer is **not** declared in Terraform — it was added out-of-band, which is enough to break refresh permanently. There is also no way to express an Enterprise Team in the `reviewers` block (`users`/`teams` only), so it cannot be adopted into config as a workaround.
```

### Steps to Reproduce

1. On GitHub Enterprise Cloud with data residency + Enterprise Managed Users, create an environment with required reviewers via Terraform (users and/or org teams).
2. Add an **Enterprise Team** as an additional required reviewer via the UI or `PUT /repos/{owner}/{repo}/environments/{environment_name}`.
3. Run `terraform plan` → the error above. It persists until the Enterprise Team reviewer is removed.

### Debug Output

`GET /repos/{owner}/{repo}/environments/{environment_name}` returns a reviewer whose top-level `type` is `BusinessTeam`, with a nested reviewer object of `type: "enterprise"`:

```json
{
"type": "BusinessTeam",
"reviewer": {
"name": "",
"id": 2418211,
"slug": "ent:",
"privacy": "closed",
"type": "enterprise",
"enterprise_id": 23181,
"permission": "pull",
"parent": null
}
}
```

For contrast, a normal user reviewer in the same response is `"type": "User"`.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start with the github_repository_environment read/refresh path and the environments API response from GET /repos/{owner}/{repo}/environments/{environment_name}. Trace how required reviewers with type "BusinessTeam" are decoded, including the possible corresponding data source. Done means terraform plan/apply refreshes successfully when an Enterprise Team reviewer is present, either by preserving it in state or ignoring it safely.

Written by the indexing model from the issue text.