Add support for project setting `protect_merge_request_pipelines`
### Description
GitLab 18.x added a new project setting **"Allow merge request pipelines to access protected variables and runners"** (Settings → CI/CD → Variables).
The REST API field `protect_merge_request_pipelines` (boolean) on `GET/PUT /projects/:id` was exposed via [gitlab-org/gitlab\!216943](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/216943) (merged 2026-02-17).
This field is **not yet present** in this Go SDK (checked `projects.go` on `main`, neither in `Project` nor `EditProjectOptions`).
### Why this matters
This blocks downstream consumers from managing the setting programmatically — notably the [`gitlab-org/terraform-provider-gitlab`](https://gitlab.com/gitlab-org/terraform-provider-gitlab) provider (tracking issue: gitlab-org/terraform-provider-gitlab#6818).
### Proposed change
Add the field to both structs:
```go
// in Project (GET response)
ProtectMergeRequestPipelines bool `json:"protect_merge_request_pipelines"`
// in EditProjectOptions (PUT/PATCH)
ProtectMergeRequestPipelines *bool `url:"protect_merge_request_pipelines,omitempty" json:"protect_merge_request_pipelines,omitempty"`
```
Also `CreateProjectOptions` if the field is accepted at project creation (per GitLab API docs — needs verification).
### Implementation Guide
- Follow the `CONTRIBUTING.md` guide for setting up your local development environment.
- Clone the community fork of this project.
- Add a new attribute called `ProtectMergeRequestPipelines` to the `Project`, `EditProjectOptions` and `CreateProjectOptions` structs in `projects.go`
- Create a unit test in `projects_test.go` testing the new fields.
- Create an integration test in `gitlab_test/projects_integration_test.go` testing creating and updating a project with the new field.
### Acceptance
- [ ] Field added to `Project` struct.
- [ ] Field added to `EditProjectOptions` (and `CreateProjectOptions` if applicable).
- [ ] Unit test covering JSON (de)serialization.
- [ ] Integration test coverage real API usage
I'm happy to open the MR if accepted.
issue