🐛 Clean up resources when the AppFixture fails to initialize

[flovouin]

📝 Description of the PR

When the initialization of an AppFixture failed, the fixture was left in an inconsistent state: it was already considered active, although the application had not been created yet. Any subsequent lifecycle call passed its state check and then failed on the missing application, replacing the original error with a TypeError. Because delete threw before reaching the fixtures, none of them were ever cleaned up, and the resources they had allocated (temporary databases, subscriptions, clients) stayed alive past the end of the test file.

AppFixture.init now releases everything it has allocated when it fails: the application is closed and the fixtures that did initialize are deleted, before the original error is rethrown. The fixture becomes active only once the application is fully initialized, and it transitions to a failed state otherwise, in which clear and delete are no-ops. All fixtures are also initialized to completion even when one of them fails, so that a slower fixture cannot allocate resources after the cleanup has run.

📋 Check list

• 🧪 Unit tests have been written.
• 📝 Documentation has been updated.