tests: add unit coverage for `scripts/docs-link-check.mjs`

[IgorShevchik]

Context

scripts/__tests__/ has unit tests for docs-lint.mjs (docs-lint.test.mjs) and docs-typecheck.mjs (docs-typecheck.test.mjs), but the third script in the docs-lint pipeline — scripts/docs-link-check.mjs — has zero unit coverage.

This came up during PR #116 review: a regression where 43 internal links silently rotted was not caught by the existing link-check. Even after we add a lint rule (#new-issue-from-PR-116), the rule's own behaviour should be pinned by tests.

Proposal

Create scripts/__tests__/docs-link-check.test.mjs modelled after the existing docs-lint.test.mjs fixture pattern (scripts/tests/docs-lint.test.mjs:132-177).

Suggested test cases:

• Internal /docs/... paths
  • Resolves to a real page → passes.
  • Resolves to a missing page → reports as broken.
  • Resolves to a directory with 0.index.md → passes.
• Heading fragments (#some-section)
  • Matches a real heading → passes.
  • Matches a slugged heading with -1/-2 suffix due to collision → passes.
  • References a missing heading → reports as broken.
• Frontmatter links: - to: /docs/...
  • Real internal target → passes.
  • Missing internal target → reports as broken.
• External URLs
  • https://example.com/... ignored entirely.
  • https://github.com/bitrix24/b24jssdk/blob/main/<path> either ignored (current behaviour) or checked (once #new-issue lands).

Acceptance criteria

• scripts/__tests__/docs-link-check.test.mjs exists with at least the cases above.
• pnpm run docs:lint:test runs them (the script already globs scripts/__tests__/**/*.test.mjs).
• CI passes on main and would catch a future regression where a to: field rots.

Related

• PR docs: fix stale skills links + refresh examples landing page #116
• tests: cover scripts/docs-link-check.mjs with unit tests #71 (sister request to expand docs-lint helper coverage)