[web] Cleanup plans-2026-phase-1 - phase 4: rest of backend (#34771)

* Remove `plans-2026-phase-1` from subscription module

* Remove code related to `plans-2026-phase-1`

* Remove `SplitTestUserGetter.getProjection`

* Update tests

* Remove last mentions of `plans-2026-phase-1`

* In test, wait for `_updateCustomerIoSubscriptionProperties` to be done

* Remove `purchaseAddon` test

* Remove AI-addon tests

* Fix `_quotaTierToAllowance` throwing on zero-grant tiers

`!quota` treated `0` as invalid, so `basic` (grant 0) threw. Check `typeof quota !== 'number'` alone. Unmasked once branch removed the split-test branch that skipped this path.

* Extract `waitForCustomerIoSync` helper in FeaturesUpdater tests

* https://github.com/overleaf/internal/pull/34771#discussion_r3552882875

GitOrigin-RevId: 0fe0e41b7a4105a3c4739214e2fffb77e91dbc42

services/web/app/src/Features/Project/ProjectController.mjs

@@ -492,7 +492,6 @@ const _ProjectController = {
       'wf-enable-freemium-super-complete',
       'wf-enable-super-complete-promotion',
       'wf-rebrand',
-      'plans-2026-phase-1',
       'testing-ai-usage',
       'wf-fake-non-english-suggestions',
       'editor-tabs',