Fix GitLab squash merge misclassification when squash_commit_sha is null

[jaivardhan-bhola]

Issue

GitLab issue #2079 - When GitLab returns squash: true without squash_commit_sha (older self-managed instances like GitLab 12.4.2), the squash commit's authorship is lost because the code falls back to the outer merge commit which has two parents and gets classified as a "simple merge"

Changes

• Added validate_squash_candidate() function that infers the squash commit from merge commit parents by tree matching
• Added fallback path in get_gitlab_ci_context() that attempts to infer the squash commit when squash_commit_sha is null
• Added 4 new test cases: valid squash candidate, reversed parent order, non-merge rejection, tree mismatch rejection
  Behavior: When squash: true and squash_commit_sha: null, the code now attempts to identify the generated squash commit S from the merge commit's parents. If successful, authorship is preserved for S. If not, a warning is emitted and the existing fallback is used.

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[CLAassistant]

All committers have signed the CLA.

[jaivardhan-bhola]

@svarlamov please review