Add direct stale-fallback coverage for news updater main flow

[Copilot]

The news updater’s stale-output fallback was only covered indirectly. This change adds direct main()-level coverage for the branch that decides whether an all-source failure should preserve the existing news.json or fail the run.

• What changed

  • Added direct tests for scripts/update_news_feed.py::main()
  • Covered the success path where:
    • all news sources fail
    • --allow-stale-on-failure is enabled
    • an existing output file is present
    • the command returns 0 without rewriting the file
  • Covered the failure path where:
    • all news sources fail
    • --allow-stale-on-failure is enabled
    • no existing output file is present
    • the command returns 1

• Why this matters

  • Locks in the CLI behavior the workflow depends on
  • Prevents regressions where stale fallback silently stops working even though lower-level helpers still pass

• Test shape

  with (
      mock.patch.object(MODULE, "parse_args", return_value=args),
      mock.patch.object(MODULE, "build_payload", return_value=payload),
  ):
      result = MODULE.main()

• Scope

  • Test-only change in scripts/update_news_feed_test.py
  • No production logic changes