Fix: Preserve session data during impersonation

[insign]

This PR prevents the application from completely flushing the user's session when starting or stopping an impersonation. This is really useful because it preserves the user's workflow and existing session data (like shopping carts or form states) while still switching their authenticated context.

Changes:

• Removed Session::invalidate(), Session::regenerateToken(), and Session::flush() from logoutAndDestroySession().
• Added tests/SessionFlushTest.php to ensure unrelated session data is retained across impersonation changes.

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PR created automatically by Jules for task 174245231779582888 started by @insign

[google-labs-jules]

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