ci: soften authorship label auth denials

[haasonsaas]

Summary

• Treat GitHub API 401/403 denials during authorship label maintenance/application as warnings instead of failing the shared reusable workflow.
• Keep non-auth label API failures hard-failing, and keep the required Maestro trailer gate after label application.
• Add a workflow guard test so the label step remains best-effort while trailer enforcement still runs.

Source signal

• evalops/deploy PR #4395 label job failed at https://github.com/evalops/deploy/actions/runs/26332969922/job/77521989046 with gh: Bad credentials (HTTP 401) while applying the authorship label.

Test Plan

• ruby -Itest -e 'ARGV.each { |path| require "./#{path}" }' test/*_test.rb\n- actionlint\n- git diff --check\n\nRollback\n- Revert this PR to restore hard failures for authorship label API auth/permission denials.