Add support for project setting `protect_merge_request_pipelines`

Gatla Vishweshwar Reddyrequested to merge
gitlab-community/gitlab-org/api/client-go:feature/protect-merge-request-pipelines-2262 into main

## What does this MR do and why?

Adds the `protect_merge_request_pipelines` field to the `Project`, `CreateProjectOptions`, and `EditProjectOptions` structs.

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 was exposed via gitlab-org/gitlab!216943 (merged) (merged 2026-02-17) but was not yet present in this Go SDK.

Closes #2262 (closed)

## Changes

  • Add `ProtectMergeRequestPipelines bool` to `Project` struct (GET)

- Add `ProtectMergeRequestPipelines *bool` to `CreateProjectOptions`

- Add `ProtectMergeRequestPipelines *bool` to `EditProjectOptions`

- Add unit tests with Gherkin GIVEN/WHEN/THEN comments covering deserialization of true and false values

## How to set up and validate locally

```go

project, _, err := client.Projects.EditProject(pid,

&gitlab.EditProjectOptions{

ProtectMergeRequestPipelines: gitlab.Ptr(true),

})

Merge request reports