Merging to release-5.12: DX-2361: Add GitHub Actions to validate external URLs and internal anchor fragments (#1865)

[buger]

DX-2361: Add GitHub Actions to validate external URLs and internal anchor fragments (#1865)

• DX-2361: Add GitHub Actions to validate external URLs and internal anchors

• Extend validate_mintlify_docs.py with --check-anchors flag that extracts
  GFM-slugified heading anchors, {#custom-id} syntax, and <a id/name> elements
  from target MDX files and verifies every internal #fragment resolves
• Update validate-docs.yml to run anchor check on every PR
• Add check-external-links.yml workflow: weekly scheduled (Mon 07:00 UTC) and
  manual-trigger run that HEAD-checks all external HTTP/HTTPS URLs

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• DX-2361: Run external link check on every pull request

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• DX-2361: Fix exit code not failing on broken external links

External link failures were reported but never factored into the
exit code, so CI was passing despite 404s. Added has_broken_external
check and a summary line for external link results.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• DX-2361: Switch external link checker to lychee

Replace sequential Python/requests approach with lycheeverse/lychee-action.
Lychee runs checks in parallel, has built-in retry logic, and is purpose-built
for link checking — much faster on a repo with 1500+ external URLs.

Add lychee.toml to exclude tyk-owned domains, localhost, placeholder URLs,
and sites known to block bots (LinkedIn, Facebook). Accept 429 as non-broken
to handle rate-limited responses gracefully.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• Fix mirror-pr workflow failing on PR bodies with special characters

GitHub Actions expands ${{ }} expressions before the shell runs, so
backticks and $ signs in PR bodies were injected raw into the script
and interpreted as shell command substitution.

Fix: write static content via a single-quoted heredoc (no shell
expansion), then append the PR body via printf with an env var
($PR_BODY). Shell variables accessed as "$VAR" are never
re-interpreted, making any PR body content safe.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• Fix lychee checking internal/relative links

Add scheme = ["https", "http"] to lychee.toml so lychee only attempts
to check http/https URLs. Without this, lychee tries to resolve
root-relative paths (/img/..., /page/...) as file URIs and fails.
Internal links are already validated by validate-docs.yml.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• Fix lychee erroring on root-relative paths

scheme = ["https", "http"] didn't help because lychee fails to build
the URL before the scheme filter runs. Set base = "https://tyk.io" so
root-relative paths (/img/..., /page/...) are resolved to
https://tyk.io/... and then silently skipped by the existing tyk.io
exclude rule, rather than producing "cannot convert path to URI" errors.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

• Fix lychee config: base -> base_url

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

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Co-authored-by: Claude Sonnet 4.6 noreply@anthropic.com

[github-actions]

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