fix: Change GitLab UpdateStatus Function to get Pipeline from Commit rather than the Merge Request

[X-Guardian]

what

Change the GitLab Client UpdateStatus function to get the pipeline ID from the commit that was used to trigger the Atlantis run, rather than the head pipeline of the Merge Request.

why

• Fixes GitLab Pipelines Status Updates fail when a Commit is Pushed to an MR Branch whilst Atlantis is Processing on the same MR #5032
• Fixes Atlantis - Gitlab integration failing when updating MR with new commits on version v0.29.0 #4950

tests

Tested locally and unit tests updated

I also refactored all the GitLab Client Update Status tests to use proper JSON handling.

References

• Supersedes fix: link latest commit to the correct pipeline #4865
• Supersedes fix: dont set commit status for old commits to new pipelines #4784

[jamengual]

@lukemassa do you have time to take a look?

[X-Guardian]

Once the container image has finished building from this PR, I will test it further on my company's non-prod Atlantis instance.

[jamengual]

Once the container image has finished building from this PR, I will test it further on my company's non-prod Atlantis instance.

did you have a chance to test this?