fix(tests): symfony 8.1 compat

[soyuka]

Summary

Restores HttpCache / Hydra / JsonApi / Symfony Bundle PHPUnit jobs and PHPStan against Symfony 8.1.0:

• ResponseHeaderBag and ParameterBag became final in Symfony HttpFoundation 8.1.0. createMock() on a final class triggers a fatal in PHPUnit 12. Swap the mocks in AddTagsProcessorTest, HydraLinkProcessorTest, JsonApiProviderTest and MercureLinkProcessorTest for real instances and assert on bag state after the call. Behaviour identical, assertions tightened (now verify the actual stored value instead of expects(...)->method('set')).
• Symfony Console 8.1.0 removed Application::add(). The fallback branch in DebugResourceCommandTest::getCommandTester() is unreachable on Symfony ≥ 7.4 but must be kept for Symfony 6.4 support. Add a targeted phpstan.neon.dist ignore (reportUnmatched: false) so analysis passes on both Symfony 6.4 and 8.1.

Test plan

• vendor/bin/phpunit src/HttpCache/Tests src/Hydra/Tests src/JsonApi/Tests tests/Symfony/Bundle/Command/DebugResourceCommandTest.php tests/Symfony/State/MercureLinkProcessorTest.php — green locally on Symfony 7.4
• vendor/bin/phpstan analyse — no errors
• CI green on Symfony 8.1.0 across the four PHPUnit components (http-cache, hydra, json-api, state) and the PHPStan job