Add support for project setting `protect_merge_request_pipelines` (!2896) · Merge requests · GitLab.org / API / client-go

## 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),

})

Add support for project setting `protect_merge_request_pipelines`

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