Disable credential persistence for safe-output checkouts - #64496

Safe-output checkouts persisted credentials unnecessarily, causing authentication failures with Git 2.55. Subsequent fetch and push operations already use the dedicated Git credential configuration step.

• Checkout generation

  • Emit persist-credentials: false for safe-output PR checkouts.
  • Stop injecting the push token into actions/checkout.
  • Continue configuring fetch/push credentials after checkout.

• Regression coverage

  • Require credential-free checkouts for create-PR and push-to-PR-branch outputs.
  • Verify the safe-output token still reaches the Git credential configuration step.

- name: Checkout repository
  uses: actions/checkout@v7
  with:
    persist-credentials: false

- name: Configure Git credentials
  run: bash "${RUNNER_TEMP}/gh-aw/actions/configure_git_credentials.sh"

• Fixes safe_outputs checkout sets persist-credentials: true and fails on git 2.55 (could not read Username) #64483