Backport #2518 to releases/v2–v6: skip unsafe PR check on default checkout

[Copilot]

Default self-checkouts (same repo, no explicit ref) were incorrectly triggering the assertSafePrCheckout guard on pull_request_target events when the event SHA matched the PR head SHA — a false positive introduced with the allow-unsafe-pr-checkout feature.

Changes

• src/input-helper.ts — Wrap assertSafePrCheckout with an isDefaultCheckout guard; skip the check when the caller hasn't customized repository or ref:

  const isDefaultCheckout = isWorkflowRepository && !core.getInput('ref')
  if (!isDefaultCheckout) {
    unsafePrCheckoutHelper.assertSafePrCheckout({ ... })
  }

• __test__/input-helper.test.ts — Two new tests: default self-checkout on fork pull_request_target succeeds; explicit fork repository is still refused.
• dist/index.js — Rebuilt artifact (v4–v6 via ncc build; v2–v3 patched directly due to pre-existing uuid/v4 ncc bundling incompatibility on those branches).

Applied identically to five branches: copilot/backport-2518-releases-v6, v5, v4, v3, v2 — each branched from its respective releases/vN base.