feat(www) Add scaffolding for WWW E2E tests and CI check (#48861)

Closes DOCS-1278

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Feature. Adds E2E test scaffolding and a CI check for the marketing
site.

## What is the current behavior?

Closes [FE-4047](https://linear.app/supabase/issue/FE-4047).

The marketing site has no E2E coverage. Docs has a suite in `e2e/docs`,
but its
runner, git helpers and axe reporting are private to that package, so a
second
site cannot reuse them.

## What is the new behavior?

* **A www suite scoped to changed content.** Changed `.mdx` files in
`_blog`,
`_events`, `_customers` and `_alternatives` map to the URLs they render.
Pages
with `disable_page_build: true` are skipped because they 404 by design.
Capped
at 20 pages. Enforces `heading-order` and `page-has-heading-one`,
matching docs.
* **`e2e/shared` The docs site is also static with similar needs. This
folder shares the docs logic with www.
* **A CI check that is safe to mark required.** Path scoping lives in a
`Detect changed paths` step rather than a `paths:` trigger, so the check
  reports on every pull request instead of being skipped.
`waitForVercelDocsPreview.js` becomes `waitForVercelPreview.js`, shared
by both
  workflows.

## How the check behaves

The job always reports a check run, so it is safe to mark required. Path
scoping
happens in a step rather than a `paths:` trigger, which would leave
non-www pull
requests waiting on a check that never reports.

| Case | Behavior |
| --- | --- |
| Fork pull request adds new pages | Passes without testing. The Vercel
wait is gated on `head.repo.full_name == github.repository`, so forks
resolve no preview URL. The job emits a `::warning` and a job summary
containing a ready-to-run `gh workflow run www-e2e.yml` command with the
resolved page paths, so a maintainer can run it against the preview. |
| Vercel preview times out or fails | Passes without testing. The wait
step is `continue-on-error: true`, so a 900s timeout or a failed
deployment leaves the URL unset and the suite skips. Vercel's own
`Vercel – zone-www-dot-com` check already reports the failure. |
| Draft pull request | Job does not run at all, gated at the job level
on `pull_request.draft == false`. `ready_for_review` is in the trigger's
`types`, so marking it ready runs the check. |
| Another app changed, www untouched | Job runs and every step skips.
The `www` filter matches only the four content directories, `e2e/www`,
`e2e/shared`, the lockfile, and this workflow. |
| Only the harness changed | Passes without testing. Scope resolves to
zero pages, and the Vercel wait is additionally gated on `www_app`, so
it does not wait for a preview Vercel skipped. |
| No preview resolves, any reason | Skips rather than falling back to
production. Production does not serve pages the pull request adds, so
testing it would fail a valid change. |

### Not covered

Changes to `apps/www` components and routes do not trigger this check —
only the
four content directories do. A follow-up can check global components
such as the navigation and the footer.

## Manual testing

1. Start the site: `pnpm dev:www`
2. Run `pnpm e2e:www` with no www content changed. It should resolve
zero pages
   and skip Playwright, not fail.
3. Touch a post, then run `pnpm e2e:www` again:
`echo "" >> apps/www/_blog/2024-01-01-some-post.mdx`. The resolved
`/blog/...`
   path should be listed before Playwright starts.
4. Run against production with no local server:
`PLAYWRIGHT_BASE_URL=https://supabase.com
WWW_E2E_PAGE_PATHS=/blog/postgres-language-server pnpm e2e:www`
5. Point step 4 at a page with a known heading problem. The failure
should name
   the rule, the CSS selector and the markup.
6. Confirm docs still passes on the shared runner: `pnpm dev:docs`, then
   `pnpm e2e:docs`

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added WWW end-to-end testing for affected content pages, including
accessibility checks.
* Added standard and full-site test commands, configurable preview
testing, and failure reports.
* Added shared utilities for page discovery, accessibility scanning, and
test execution.

* **Documentation**
* Documented WWW test setup, coverage, debugging, CI behavior, and
running checks against production or preview environments.

* **Improvements**
* Updated documentation test workflows to better identify affected
changes and handle preview environments.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/docs-e2e.yml

@@ -55,12 +55,8 @@ jobs:
               - 'apps/docs/content/guides/**/*.mdx'
               - 'apps/docs/content/troubleshooting/**/*.mdx'
               - 'apps/docs/content/_partials/**'
-              - 'e2e/docs/features/**'
-              - 'e2e/docs/utils/**'
-              - 'e2e/docs/scripts/**'
-              - 'e2e/docs/playwright.config.ts'
-              - 'e2e/docs/package.json'
-              - 'e2e/docs/tsconfig.json'
+              - 'e2e/docs/**'
+              - 'e2e/shared/**'
               - 'pnpm-lock.yaml'
               - '.github/workflows/docs-e2e.yml'
             docs_app:
@@ -75,6 +71,7 @@ jobs:
           fetch-depth: ${{ github.event_name == 'pull_request' && '0' || '1' }}
           sparse-checkout: |
             e2e/docs
+            e2e/shared
             scripts
             patches
             apps/docs/content/guides
@@ -138,17 +135,18 @@ jobs:
       # times out polling that API even though the preview builds fine.
       # Poll the "Vercel – docs" commit status instead — Vercel keeps posting
       # those — then resolve the deployment it points to via Vercel's own API
-      # to get the actual preview URL. See scripts/waitForVercelDocsPreview.js.
+      # to get the actual preview URL. See scripts/waitForVercelPreview.js.
       # A Vercel failure or timeout is not the author's problem, and the required
       # "Vercel – docs" check already reports it. Resolve no URL and skip below.
       - name: Wait for Vercel docs preview
         if: steps.scope.outputs.skip == 'false' && github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository && steps.changes.outputs.docs_app == 'true'
         id: deployment
         continue-on-error: true
-        run: node scripts/waitForVercelDocsPreview.js
+        run: node scripts/waitForVercelPreview.js
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          VERCEL_STATUS_CONTEXT: 'Vercel – docs'
           VERCEL_TOKEN: ${{ secrets.VERCEL_TOKEN }}
           VERCEL_TEAM_ID: ${{ secrets.VERCEL_TEAM_ID }}