Fix bug with inner .Count(predicate) translation

[SergeiPavlov]

The new DataObjects from Xtensive (starting 7.3.12 in our version numbering)
cannot translate inner .Count(predicate) expressions like following:

session.Query.All<Promotion>().GroupBy(p => p.Id).Select(g =>
      g.Count(x => x.CampainName == null)
    )

See the discussion: https://servicetitan.slack.com/archives/CLM7XAVLK/p1776720202265489?thread_ts=1776709176.616779&cid=CLM7XAVLK

This PR fixes it, translating as an equivalent sequence .Where(predicate).Count()

Also:

• Add a test

[snaumenko-st]

@SergeiPavlov I'm going to fix the root cause of this bug in the upcoming PR. And it's not something new.
upd: #478 is a replacement