test: stop mercure tests hitting a public hub

[soyuka]

What

Mercure tests in the general PHPUnit matrix published to the public https://demo.mercure.rocks. When that third party hiccups, jobs go red — it caused 2 of the reds on 4.3 today (PHPUnit (PHP 8.3), PHPUnit (PHP 8.5) (Symfony 8.1)), and a plain re-run went green with no code change.

Root cause

The fixture app declared two mercure hubs on the same URL:

mercure:
    hubs:
        default: { url: '%env(MERCURE_URL)%', ... }
        debug:   { url: '%env(MERCURE_URL)%', ... }

MercureBundle registers one messenger handler per hub, and both are tagged messenger.message_handler for the same Update message:

• mercure.hub.default.message_handler → mercure.hub.default.traceable → TestHub ✅
• mercure.hub.debug.message_handler → real Hub ❌

So every async mercure update was published twice — once through TestHub (recorded, and what the assertions read) and once straight over the network through an unguarded hub. TestHub was also a decorator rather than a fake: publish() recorded and forwarded, even though nothing anywhere consumes its return value.

The debug hub existed only to be TestHub's inner; nothing targets it by name.

Change

• Collapse the second hub into a plain app.mercure.inner_hub service, so a single messenger handler remains and the duplicate publish is gone.
• TestHub gains $publishToHub (default false): it always records, and forwards only when asked.
• config_mercure.yml (APP_ENV=mercure) sets app.mercure.publish_to_hub: true, so the dedicated PHPUnit (Mercure) job keeps real end-to-end coverage against its local hub container.
• MERCURE_URL now defaults to http://localhost:1337/.well-known/mercure — the hub that job already runs — instead of a public URL.
• New TestHubTest locks the invariant in, so re-introducing forwarding fails loudly instead of going quietly flaky.

No changes to .github/workflows/ci.yml were needed.

Two assertions hardcoded demo.mercure.rocks and were updated. Note testNoDiscoveryLinkOnNonMercureEndpoint previously asserted the absence of a .../hub URL that was never emitted, so it was vacuous; it now asserts the absence of the real discovery link.

Verification

• tests/Functional/Mercure/ + SubscriptionTest: 9 tests, 84 assertions, green fully offline (before this change they died at 22 assertions).
• app.mercure.publish_to_hub resolves true under APP_ENV=mercure, false under test.
• php-cs-fixer: 0 fixes. PHPStan: clean on the changed files (local run shows only the usual Doctrine\ODM\MongoDB class.notFound noise on untouched lines).

I stopped a local full-suite run partway for thermal reasons — CI is the authority on the full matrix here.