[BUG]: github_branch_protection fails with 'Resource not accessible by integration' when app node_id is in dismissal_restrictions

### Expected Behavior

When a GitHub App node ID is included in `dismissal_restrictions` of `github_branch_protection`, the provider should be able to read and manage the branch protection rule, including setting app node IDs alongside team node IDs.

### Actual Behavior

The provider fails with `Resource not accessible by integration` during both **plan** and **apply** when:
1. An app node ID is specified in `dismissal_restrictions` in the HCL config, OR
2. An app has been added to `dismissal_restrictions` via the GitHub API externally (state refresh fails)

This makes it impossible to use Terraform to manage branch protection rules that include GitHub Apps in dismissal restrictions.

### Terraform Version

```
Terraform v1.6.0
+ provider registry.terraform.io/integrations/github v6.x
```

### Affected Resource(s)

- `github_branch_protection`

### Terraform Configuration Files

```hcl
data "github_team" "maintainers" {
for_each = {
for team in var.teams : team.team_slug => team
if team.role == "maintain" || team.role == "admin"
}
slug = each.key
}

resource "github_branch_protection" "this" {
for_each = toset(var.protected_branches)
repository_id = github_repository.this.node_id
pattern = each.value

required_pull_request_reviews {
dismiss_stale_reviews = false
require_code_owner_reviews = true
required_approving_review_count = 1
restrict_dismissals = true
dismissal_restrictions = concat(
[for team_data in data.github_team.maintainers : team_data.node_id],
["A_kwPOAAELEM4AECna"] # GitHub App node ID
)
}

enforce_admins = false
}
```

### Steps to Reproduce

1. Create a `github_branch_protection` resource with `restrict_dismissals = true`
2. Include a GitHub App node ID in `dismissal_restrictions` alongside team node IDs
3. Run `terraform plan` or `terraform apply`

**Alternative reproduction:**
1. Have a working `github_branch_protection` with only team node IDs in `dismissal_restrictions`
2. Externally add a GitHub App to `dismissal_restrictions` via the GitHub REST API:
```
PATCH /repos/{owner}/{repo}/branches/{branch}/protection/required_pull_request_reviews
{ "dismissal_restrictions": { "teams": [...], "apps": ["my-app-slug"] } }
```
3. Run `terraform plan` — fails during state refresh

### Error Output

```
Error: Resource not accessible by integration

with github_branch_protection.this["main"],
on repo.tf line 158, in resource "github_branch_protection" "this":
158: resource "github_branch_protection" "this" {
```

### Additional Context

- The GitHub REST API supports apps in `dismissal_restrictions` and works correctly with user PATs
- The GitHub App used by the Terraform provider has `administration:write` permission on the org
- The error occurs even when the app node ID in `dismissal_restrictions` belongs to a different (non-Terraform) app that is installed on the repository
- This appears to be a provider-level limitation where the GraphQL mutation used for branch protection rules doesn't support app actors in `dismissalRestrictionsActorIds`, or the provider doesn't properly translate app node IDs for the mutation

### Workaround

Currently the only workaround is to manage the app entry in `dismissal_restrictions` via the GitHub REST API outside of Terraform, and ensure Terraform never touches repos where an app has been added. This is fragile and creates state drift.