Run the Cypress suite with pretty permalinks, fixing the Preview panel poll spec

[galbus]

Problem

tests/cypress/e2e/block-editor/preview-panel.cy.js fails on main:

1) Block Editor: Preview panel
     waits for a still-processing voice-customised preview instead of embedding it early:
   CypressError: Timed out retrying after 5000ms: `cy.wait()` timed out waiting `5000ms`
   for the 1st request to the route: `statusCheck`. No request ever occurred.

Cause

The test environment was running plain permalinks, which almost no real site does.

The workflow already sets a permalink structure before starting Cypress:

- name: Set permalinks
  run: |
    wp rewrite structure '/%postname%/' --hard

…but the first spec's setupDatabase task runs wp reset reset --yes (wp-reset), which
restores WordPress defaults and wipes it. So from spec 1 onwards, every spec ran against
plain permalinks, and that step was dead configuration.

Under plain permalinks get_rest_url() returns the query-string form, so the poll in
src/editor/components/play-audio/hooks.js — apiFetch( { path } ) — requests:

/index.php?rest_route=%2Fbeyondwords%2Fv1%2Fprojects%2F9969%2Fcontent%2F<id>&_locale=user

The spec's glob (**/beyondwords/v1/projects/<id>/content/<id>*) has no slashes to match
against, so the intercept never fired, cy.wait( '@statusCheck' ) timed out, and the poll
hit the real endpoint instead of the stub. Classic-editor specs use the same glob shape and
pass because that code builds URLs as restUrl + 'beyondwords/v1/…', keeping the slashes
literal.

Fix

Test-only — no source change.

• Set the permalink structure in setupDatabase, after the reset that was wiping it, so
  the suite exercises the /wp-json/… routes real sites use. The dead workflow step is
  removed, with a comment recording what owns the structure now.
• Add tests/cypress/scripts/router.php for the CI web server. CI serves the site with
  php -S, which has no rewrite support, so pretty URLs would 404 wholesale; the router
  hands non-file requests to index.php the way Apache's .htaccess fallback does.
• Match the poll with a plain path glob again, now that the URL has literal slashes in
  it. No separator-encoding workaround in the spec.
• Stub the player SDK via window:before:load instead of intercepting the CDN script.
  With the namespace already defined the plugin never appends the script, so the assertions
  no longer depend on intercepting a cross-origin request that an earlier spec may have left
  in the browser cache — which is how the real player ended up running in the CI failure.

One other spec depended on the old structure: plugins/amp.cy.js built its AMP URL as
`${ url }&amp=1`, which only parsed while every post URL already carried a query string
(/?p=123). It now sets the query var through URL/searchParams, so it is correct under
either structure.

Note that embeds a voice-customised preview once it has finished processing had never
actually executed in CI: cypress-fail-fast skipped it behind the failing test. It runs and
passes here, so both sides of the poll-then-embed behaviour are now covered.

Verification

npx cypress run --browser chrome --spec 'tests/cypress/e2e/block-editor/preview-panel.cy.js'
→ 3 passing

Because the permalink change is environment-wide, the full suite was also run locally with
fail-fast disabled (--expose failFastEnabled=false) so that every permalink-related failure
surfaced rather than being skipped behind the first one:

27 specs, 231 tests → 221 passing, 8 pending, 2 failing
✖ plugins/amp.cy.js   (both failures — the `&amp=1` URL above)

plugins/amp.cy.js passes with that spec fixed; no other spec was affected.

The router was verified standalone against a stub docroot: /, /my-post/ and
/wp-json/…?_locale=user reach WordPress, while /style.css and /wp-admin/ are served
directly.

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