fix #95 Detach the written log entities without detaching the process execution

[njoubert-cleverage]

Description

Fixes #95.

• DoctrineProcessHandler::flush(): keeps the LogRecord entities created from the buffered records and detaches them after the flush; the loop detaching the Monolog records (no effect) is removed
• LogRecord::$processExecution: cascade reduced from all to persist, so that detaching a log entity does not detach the current ProcessExecution (which the next ProcessExecutionManager::save() would insert again, silently). ORM level only: no schema change, no migration (doctrine:schema:update --dump-sql output identical with and without this change)

Tests:

• DoctrineProcessHandlerEntityManagerTest (new, entity manager of the test application, SQLite): 3 batches of 100 records; after each flush, no LogRecord left in the identity map and the process execution still managed; then save() updates the execution instead of inserting it again (a single finish row, 300 log records attached to it)
• DoctrineProcessHandlerTest: only the persisted log entities are detached; nothing is detached without a current process execution

Checked:

• bundle (Symfony 8.1, PHP 8.5, doctrine/orm 3.7): 330 tests OK. Without the fix, the new test fails (log entities still managed); with the detach only (cascade all kept), it fails too (process execution no longer managed). PHPStan, PHP-CS-Fixer, Rector OK
• process-bundle-demo (Symfony 7.4): demo.logger ends with the finish status and its logs attached

Requirements

• Documentation updates
  • Reference
  • Changelog
• Unit tests

Breaking changes

None in practice: only an application removing or refreshing LogRecord entities and relying on the cascade to their process execution would be affected (already a dangerous behaviour: with onDelete: CASCADE, removing one log record removed the execution and all its logs).

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