Add latest_pipeline endpoint (#656)

[mvanhorn]

Closes #656.

Wraps the GitLab REST endpoint `GET /projects/:id/pipelines/latest` (docs).

Usage

```ruby
Gitlab.latest_pipeline(5) # latest pipeline on the project's default branch
Gitlab.latest_pipeline(5, ref: 'develop') # latest pipeline on a specific ref
```

The method follows the same shape as the existing `pipeline` / `pipelines` methods in `lib/gitlab/client/pipelines.rb` and forwards `:ref` (and any other future query params) through `get`'s `query:` argument so callers don't have to remember which params the endpoint supports today.

Tests

`spec/gitlab/client/pipelines_spec.rb` adds two contexts:

• "without a ref option" — verifies the request URL and that the response is parsed into a `Gitlab::ObjectifiedHash`.
• "with a ref option" — verifies the `ref` query param is forwarded.

Backed by a new `spec/fixtures/latest_pipeline.json` fixture mirroring the existing `pipeline.json` shape.

I could not run `bundle exec rspec` locally — system Ruby is 2.6 and the gemspec requires Ruby 3.2. `ruby -c` confirms the source and spec parse cleanly. Happy to fix anything CI flags.

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[mvanhorn]

The Rubocop failure here isn't from this PR — I reproduced it on master at the same commit range and got the identical 6 offenses:

• lib/gitlab/shell_history.rb:33 — Style/FileOpen
• spec/gitlab/client/projects_spec.rb:549,566 and spec/gitlab/error_spec.rb:40,46,62 — RSpec/MatchWithSimpleRegex

This PR only touches lib/gitlab/client/pipelines.rb, its spec, and a fixture, none of which are flagged. 5 of 6 are autocorrectable with rubocop -a. Happy to send a separate lint-only PR for them if that's useful — kept out of this one so the endpoint change stays reviewable on its own.

[mvanhorn]

The Rubocop job here isn't from this PR. I ran it locally on this branch and on master and got the same six offences either way:

• lib/gitlab/shell_history.rb:33 -- Style/FileOpen
• spec/gitlab/client/projects_spec.rb:549,566 and spec/gitlab/error_spec.rb:40,46,62 -- RSpec/MatchWithSimpleRegex

None of them are in the three files this PR touches (lib/gitlab/client/pipelines.rb, spec/gitlab/client/pipelines_spec.rb, spec/fixtures/latest_pipeline.json) -- it looks like a newer Rubocop picked up existing code. All four Ruby test jobs pass.

I've left them alone so this PR stays scoped to the endpoint. Happy to send a separate lint-only PR if that's useful -- five of the six are autocorrectable and the File.open one wants a real look.