Clean merge_jid whenever necessary on the merge process

Merged Oswaldo Ferreira requested to merge 38476-improve-merge-jid-cleanup-on-merge-process into master

MergeRequest#merge_jid should be cleaned up whenever we hit a known error on MergeService#execute (or we finished processing). This way we can keep track if the MR is really "ongoing" or just "stuck" (stuck MRs are already being handled by StuckMergeJobsWorker).

Currently, if a an user clicks Merge and the MR is not mergeable, or no source is found, we keep stuck on merge ongoing state.


Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #38476 (closed)

Edited by Oswaldo Ferreira