Use fully-qualified `PersistedEventPosition` (`instance_name` and `stream_ordering`) when returning `RoomsForUser` to facilitate proper comparisons and `RoomStreamToken` generation.
Spawning from https://github.com/element-hq/synapse/pull/17187 where we want to utilize this change
Before:
```
$ SYNAPSE_POSTGRES=1 SYNAPSE_POSTGRES_USER=postgres SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.replication.storage.test_events
[...]
Traceback (most recent call last):
File "pypoetry/virtualenvs/matrix-synapse-xCtC9ulO-py3.12/lib/python3.12/site-packages/twisted/trial/runner.py", line 711, in loadByName
return self.suiteFactory([self.findByName(name, recurse=recurse)])
File "pypoetry/virtualenvs/matrix-synapse-xCtC9ulO-py3.12/lib/python3.12/site-packages/twisted/trial/runner.py", line 474, in findByName
obj = reflect.namedModule(searchName)
File "pypoetry/virtualenvs/matrix-synapse-xCtC9ulO-py3.12/lib/python3.12/site-packages/twisted/python/reflect.py", line 156, in namedModule
topLevel = __import__(name)
File "synapse/tests/replication/storage/test_events.py", line 33, in <module>
from synapse.handlers.room import RoomEventSource
File "synapse/synapse/handlers/room.py", line 74, in <module>
from synapse.rest.admin._base import assert_user_is_admin
File "synapse/synapse/rest/__init__.py", line 24, in <module>
from synapse.rest import admin
File "synapse/synapse/rest/admin/__init__.py", line 41, in <module>
from synapse.handlers.pagination import PURGE_HISTORY_ACTION_NAME
File "synapse/synapse/handlers/pagination.py", line 30, in <module>
from synapse.handlers.room import ShutdownRoomParams, ShutdownRoomResponse
builtins.ImportError: cannot import name 'ShutdownRoomParams' from partially initialized module 'synapse.handlers.room' (most likely due to a circular import) (synapse/synapse/handlers/room.py)
```
Otherwise things will get confused.
An alternative would be to make sure that for lagging stream we don't
return anything (and make sure the returned next_batch token doesn't go
backwards). But that is a faff.
We try and deduplicate in two places: 1) really early on, and 2) just
before we persist the event. The first case was broken due to it
occuring before the profile information was added, and so it thought the
event contents were different.
The second case did catch it and handle it correctly, however doing so
creates a redundant state group leading to bloat.
Fixes#3791
Fixes up #17239
We need to keep the spam check within the `try/except` block. Also makes
it so that we don't enter the top span twice.
Also also ensures that we get the right thumbnail length.
