Ensure Update is in session before linking relationships

[mattiaverga]

In production I have been seeing SQLAlchemy warnings like:

2025-01-06 08:12:34,735 INFO [bodhi.server][ThreadPoolExecutor-0_0] Creating new update: [<RpmBuild {'nvr': 'perl-Net-OAuth-0.30-1.el10_0', 'signed': False, 'release_id': 83, 'type': 'rpm', 'epoch': 0}>]
2025-01-06 08:12:34,736 WARNI [bodhi.server][ThreadPoolExecutor-0_0] No JSON file found for collection epel10
/usr/lib/python3.12/site-packages/bodhi/server/models.py:2134: SAWarning: Object of type <Update> not in session, add operation along 'User.updates' will not proceed (This warning originated from the Session 'autoflush' process, which was invoked automatically in response to a user-initiated operation.)
u = Update.query.filter(Update.release_id == release.id, Update.id != self.id).first()
/usr/lib/python3.12/site-packages/bodhi/server/models.py:2134: SAWarning: Object of type <Update> not in session, add operation along 'Bug.updates' won't proceed (This warning originated from the Session 'autoflush' process, which was invoked automatically in response to a user-initiated operation.)
u = Update.query.filter(Update.release_id == release.id, Update.id != self.id).first()
/usr/lib/python3.12/site-packages/bodhi/server/models.py:2134: SAWarning: Object of type <Update> not in session, add operation along 'Build.update' won't proceed (This warning originated from the Session 'autoflush' process, which was invoked automatically in response to a user-initiated operation.)
u = Update.query.filter(Update.release_id == release.id, Update.id != self.id).first()
2025-01-06 08:12:35,042 INFO [bodhi.server][ThreadPoolExecutor-0_0] Deferring working on bugs and fetching test cases to celery

This seems to happen for updates with bugs.
With this PR I'll try to add the relationships only after the update is added to the session. The tests seems happy with this, so let's try...