Fix getEffectiveProjectId to prioritize passed projectId over GITLAB_PROJECT_ID

[Copilot]

When GITLAB_ALLOWED_PROJECT_IDS is unset, getEffectiveProjectId was returning GITLAB_PROJECT_ID even when a different projectId was explicitly passed, preventing users from querying other projects via tools like get_project.

Changes

• index.ts: Reversed priority order to check projectId parameter before GITLAB_PROJECT_ID environment variable
• test/test-geteffectiveprojectid.ts: Added test coverage for all three configuration modes (no allowlist, single project, multiple projects)

Before

return GITLAB_PROJECT_ID || projectId;

After

if (projectId) {
  return projectId;
}
if (GITLAB_PROJECT_ID) {
  return GITLAB_PROJECT_ID;
}
throw new Error("No project ID provided and GITLAB_PROJECT_ID is not set");

This allows get_project and similar tools to accept any project ID/path when no allowlist is configured, while maintaining backward compatibility when projectId is omitted.

Original prompt

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This section details on the original issue you should resolve

<issue_title>getEffectiveProjectId cannot return projectId passed into the function when GITLAB_ALLOWED_PROJECT_IDS is unspecified</issue_title>
<issue_description>I want to retrieve the project ID of another project (different from GITLAB_PROJECT_ID) by the project path with namespace using get_project. It's supported by Gitlab project API: https://docs.gitlab.com/18.6/api/projects/#get-a-single-project

However, getEffectiveProjectId always returns GITLAB_PROJECT_ID when GITLAB_ALLOWED_PROJECT_IDS is unspecified. Can it have an option to return projectId passed into the function, or even let projectId have precedence over GITLAB_PROJECT_ID at https://github.com/zereight/gitlab-mcp/blob/v2.0.21/index.ts#L1307 ? Thanks.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes getEffectiveProjectId cannot return projectId passed into the function when GITLAB_ALLOWED_PROJECT_IDS is unspecified #298

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