Support model selection when requesting Copilot code review from `gh pr`

### Describe the feature or problem you’d like to solve

GitHub CLI now supports requesting Copilot code review from the terminal, which appears to address:
- cli/cli#10598

Examples documented in the changelog include:
- `gh pr edit --add-reviewer @copilot`
- `gh pr create --reviewer @copilot`

Related references:
- [GitHub changelog (2026-03-11): Request Copilot code review from GitHub CLI](https://github.blog/changelog/2026-03-11-request-copilot-code-review-from-github-cli/)
- [GitHub changelog (2026-03-05): Pick a model for @copilot in pull request comments](https://github.blog/changelog/2026-03-05-pick-a-model-for-copilot-in-pull-request-comments/)

However, I could not find any way to choose **which model** Copilot should use for the review.

I checked the current implementation around:
- [`pkg/cmd/pr/review/review.go`](https://github.com/cli/cli/blob/trunk/pkg/cmd/pr/review/review.go)
- [`pkg/cmd/pr/shared/params.go`](https://github.com/cli/cli/blob/trunk/pkg/cmd/pr/shared/params.go)
- [`pkg/cmd/pr/create/create.go`](https://github.com/cli/cli/blob/trunk/pkg/cmd/pr/create/create.go)
- [`api/queries_repo_test.go`](https://github.com/cli/cli/blob/trunk/pkg/cmd/pr/create/create.go)

and related PRs:
- #12567
- #12627
- #12854

From what I can tell, the CLI supports `@copilot` as a reviewer identity, but there does not seem to be any parameter or plumbing for passing a model name along with the review request.

### Proposed solution

If the backend/product supports model selection for Copilot code review, it would be helpful for `gh` to expose it.

Possible UX ideas:

```bash
gh pr edit 123 --add-reviewer @copilot --copilot-model gpt-5
gh pr create --reviewer @copilot --copilot-model gpt-5
```

or possibly:

```bash
gh pr edit 123 --add-reviewer @copilot:gpt-5
gh pr create --reviewer @copilot:gpt-5
```

Related issue: #10598