fix: Fix git-sha value for the GitLab when merge strategy doesn't cre…

[SergK]

Description

This PR fixes an issue where GitLab webhook payloads return null for merge_commit_sha when using fast-forward or squash merge strategies, causing build pipelines to fail. The fix changes the triggerbinding to use extensions.pullRequest.headSha instead, which correctly maps to the last_commit.id field that is always available in GitLab merge request webhooks.

Fixes #569

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which improves an existing feature or documentation)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

How Has This Been Tested?

Tested with GitLab merge request events in an environment configured with fast-forward merge strategy. Verified that the TriggerBinding now correctly extracts the commit SHA from extensions.pullRequest.headSha, which is populated by the interceptor from the last_commit.id field in the webhook payload.

Checklist:

• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Pull Request contains one commit. I squash my commits.

Screenshots (if appropriate):

N/A

Additional context

The GitLab API documentation states that merge_commit_sha returns null until the MR is merged, and remains null for fast-forward merge strategies. The last_commit.id field is always available and represents the head commit of the merge request, making it the reliable choice for triggering build pipelines on merge request events.

[SergK]

/recheck