git: add contract tests for force-push resilience, error recovery, and edge cases

[Dev-iL]

Summary

Add 12 unit tests to the Git DAG bundle provider, expanding test coverage for force-push resilience, error recovery, and edge cases. The new tests cover previously untested defensive paths.

Context

These tests were motivated by an investigation into repeated recloning behavior observed after force-pushing a tracked tag.

Force-push resilience (6 tests)

Test	What it proves
test_refresh_tag_force_pushed_to_unrelated_commit	Tag moved to orphan commit is followed
test_refresh_branch_force_pushed_to_unrelated_commit	Branch equivalent of the above
test_refresh_tag_moved_forward_and_backward	Non-monotonic tag movement across multiple refreshes
test_refresh_after_force_push_does_not_reclone	Repo.clone_from is never called during refresh
test_repeated_refreshes_after_force_push_stable	Post-force-push state doesn't degrade
test_reinitialize_reuses_repos_after_force_push	New bundle object reuses existing repos (processor restart)

Error handling (4 tests)

Test	What it proves
test_refresh_survives_upstream_tag_deletion	Deleted upstream tags persist locally; refresh succeeds
test_refresh_failure_preserves_previous_checkout	Failed fetch leaves working tree intact
test_refresh_versioned_bundle_raises	refresh() on versioned bundle raises AirflowException
test_clone_repo_invalid_repository_error_retry	Corrupted working repo triggers cleanup + retry

Edge cases (2 tests)

Test	What it proves
test_refresh_with_real_submodules_after_ref_change	Real submodule (two repos) stays intact through ref change
test_refresh_ambiguous_ref_prefers_branch_over_tag	Documents branch-over-tag preference when names collide

Notable finding

git reset --hard fails when the target tree references a different submodule commit than the current checkout, because the existing mysub/.git file triggers git's invalid-path protection (verified on git 2.43). This means refresh() with submodules=True cannot handle upstream submodule reference changes. A fix would require deiniting submodules before the reset. This is a pre-existing limitation, not introduced by this PR.

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[github-actions]

Backport successfully created: v3-2-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-2-test