fix: clamp count() at 0 when offset exceeds total (#2208)

* fix: clamp count() to limit/offset semantics at the edges

COUNT(*) ignores LIMIT/OFFSET, so CountQuery applies them in Python. Two edge
cases didn't match the limited query result:
- offset greater than the total returned a negative number (e.g.
  .offset(100).count() == -95 for 5 rows) instead of 0.
- limit(0) returned the full total instead of 0, because the limit clamp used
  a truthiness check (`if self._limit`) that skips 0.

Clamp the offset subtraction at 0 and use `self._limit is not None` so an
explicit limit(0) is honored.

* docs: reference count fix PR in changelog

* test: format count edge assertions