fix #103 Reset the entity manager when a process task has closed it

[njoubert-cleverage]

Fixes #103 (extension: entity manager closed by a process task; the cleared case was fixed by #104)

Changes

• ProcessExecutionRepository::getManaged(): a closed entity manager is reset first (ManagerRegistry::resetManager()). The entity manager service is lazy: it is reset in place, so the services holding it (repository, DoctrineProcessHandler, process tasks) get an open one, as Symfony Messenger does after a Doctrine error. New optional constructor argument ?ManagerRegistry $registry = null (@doctrine in repository.yaml), no BC break.
• DoctrineProcessHandler::flush(): without log to write, a closed entity manager is not flushed (it threw The EntityManager is closed. at the end of the process); it is reset when writing the next logs or saving the execution.
• CHANGELOG: Current process execution inserted again or left started when a process task clears or closes the entity manager #103 entry completed (unreleased).

Tests

• ProcessExecutionManagerEntityManagerTest::testEntityManagerClosedDuringTheProcess (entity manager of the test application): log, close(), log, end of the process: a single finish execution with its report, context and both logs; the entity manager service is open again (same instance).
• DoctrineProcessHandlerTest: closed entity manager without records: no flush; open one: flushed as before.
• Without the fix, the 2 new tests fail; with it, 295 tests OK. PHPStan, PHP-CS-Fixer, Rector OK.

Checked in process-bundle-demo (MySQL, doctrine-bundle 3.3, PHP 8.5, Symfony 7.4)

demo.ui.entity_manager_closed: log, DoctrineWriterTask writing an Author with a 300 characters last name (Data too long, entity manager closed by Doctrine), error_strategy: skip.

	Execution	Logs	Command
Before	started, no end date	0	Critical process failure: The EntityManager is closed.
After	finish	9 (including the SQL error)	executed successfully

With error_strategy: stop: a single failed execution with its 8 logs.

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