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

[koriyoshi2041]

Description

QuerySet.count() returns a negative number when offset() is larger than the total row count.

COUNT(*) ignores LIMIT/OFFSET, so CountQuery applies the offset in Python by subtracting it (tortoise/queryset.py):

count = list(dict(result[0]).values())[0] - self._offset

When offset > total, this goes negative. For example, with 5 rows:

await Item.all().offset(100).count()        # -> -95
len(await Item.all().offset(100))           # ->  0   (what SQL LIMIT/OFFSET actually returns)

The fix clamps the result at 0, matching the row count the equivalent query would actually return.

Motivation and Context

A count should never be negative; callers using .offset(...).count() (e.g. for pagination "remaining" math) get a nonsensical value.

How Has This Been Tested

Added test_offset_count_beyond_total in tests/test_queryset.py: with the 30-row IntFields fixture, offset(100).count() must equal 0 and match len(await ...offset(100)). It fails before the fix (-70) and passes after. Added a CHANGELOG.rst entry under 1.1.8 → Fixed.