Fix `Possible bug: Vote import failed` after aggression is enabled (#6690)

After finality started lagging on kusama around 025-11-25 15:55:40
validators started seeing ocassionally this log, when importing votes
covering more than one assignment.
```
Possible bug: Vote import failed
```

That happens because the assumption that assignments from the same
validator would have the same required routing doesn't hold after you
enabled aggression, so you might end up receiving the first assignment
then you modify the routing for it in `enable_aggression` then your
receive the second assignment and the vote covering both assignments, so
the rouing for the first and second assingment wouldn't match and we
would fail to import the vote.

From the logs I've seen, I don't think this is the reason the network
didn't fully recover until the failsafe kicked it, because the votes had
been already imported in approval-voting before this error.

---------

Signed-off-by: Alexandru Gheorghe <[email protected]>