test: enable GitHub label sync wrapper test

[ccai40359-wq]

Summary

• Enable Test_sync_gh_issue_labels1.test1 by fixing the missing input_label setup.
• Mock the outer docker wrapper so the test does not require a running Docker daemon.
• Exercise the dockerized implementation directly to verify that --backup writes the expected label backup file.

Verification

• python -m pytest dev_scripts_helpers/github/test/test_sync_gh_issue_labels.py::Test_sync_gh_issue_labels1::test1 dev_scripts_helpers/github/test/test_dockerized_sync_gh_issue_labels.py::TestLabel1::test_save_label dev_scripts_helpers/github/test/test_dockerized_sync_gh_issue_labels.py::TestLabel1::test_load_label -q -rs → 3 passed
• git diff --check → passed
• Static added-line security scan → security_findings_count 0

Closes #753