fix(test): support symfony/mercure 0.8 in test fixtures

[soyuka]

Unblocks 4.3 CI, which has produced no test signal since 2026-07-08.

What broke

symfony/mercure v0.8.0 was released 2026-08-11 and added two methods to HubInterface:

public function getProtocolVersion(): ProtocolVersion;   // v0.8.0 src/HubInterface.php:46
public function getCookieName(): string;                 // v0.8.0 src/HubInterface.php:51

tests/Fixtures/TestBundle/Mercure/TestHub.php is a decorator implementing HubInterface and did not define them:

PHP Fatal error: Class ApiPlatform\Tests\Fixtures\TestBundle\Mercure\TestHub contains 2 abstract
methods and must therefore be declared abstract or implement the remaining methods
(Symfony\Component\Mercure\HubInterface::getProtocolVersion, ...::getCookieName)

This kills the container at cache warmup, so all 20 failing jobs died before PHPUnit started — zero tests ran. It is entirely test infrastructure: nothing in src/ implements HubInterface, and no shipped code is affected.

It landed silently because symfony/mercure-bundle was declared as "*" in all four composer files — a bare wildcard on a 0.x component.

What changed

1. TestHub implements both new methods, delegating to the inner hub.
2. "symfony/mercure-bundle": "*" → "^0.4.3|^0.5" in composer.json, src/GraphQl/composer.json, src/Serializer/composer.json, src/Symfony/composer.json. All four are require-dev; the suggest entry in src/Symfony/composer.json is untouched.

The constraint deliberately spans the breaking boundary so both majors stay covered:

mercure-bundle	resolves symfony/mercure
v0.4.3	^0.6.1|^0.7 — what the lowest jobs install
v0.5.0	^0.8

ProtocolVersion does not exist before 0.8. Declaring it as a return type is safe because PHP resolves return types lazily and nothing calls these methods on older versions — but that is an assumption worth testing rather than trusting, so it was tested.

Verification

Both majors, cache warmup plus the Mercure functional tests:

symfony/mercure	ProtocolVersion present	cache:warmup	phpunit tests/Functional --filter Mercure
v0.7.2	no	OK	OK (6 tests, 30 assertions)
v0.8.0	yes	OK	OK (6 tests, 30 assertions)

php-cs-fixer: no changes. PHPStan on the changed fixture: no errors.

Note

Branches 4.4 and main carry the identical TestHub and the same "*" constraint. 4.4 last ran CI on 2026-07-12, before mercure 0.8.0, so it has not detonated yet — it will on the next push. This should merge up rather than be fixed separately.