fix: keep timezone-aware datetimes aware and UTC-normalise naive coercion

[simozzy]

Problem

DatabendDateTime.result_processor currently coerces every non-string value with an unconditional value.replace(tzinfo=None):

else:
    return value.replace(tzinfo=None)

The driver returns tz-aware (UTC) datetimes, so this does fix the naive-column case — but it has two side effects:

1. It clobbers timezone=True columns. A DateTime(timezone=True) / TIMESTAMP WITH TIME ZONE column loses its tzinfo, even though self.timezone is available (inherited from sqltypes.DATETIME) to distinguish the two cases.
2. It drops tzinfo without converting to UTC first. If the driver ever returns a tz-aware value carrying a non-UTC offset, replace(tzinfo=None) truncates the offset instead of converting it — yielding the wrong naive wall-clock (e.g. a +02:00 value comes back two hours off).

Fix

if value.tzinfo is not None and not self.timezone:
    return value.astimezone(datetime.timezone.utc).replace(tzinfo=None)
return value

• timezone=False columns still yield naive values (so the compliance suite's naive round-trips hold), but the value is normalised to UTC before tzinfo is dropped, so the wall-clock is correct regardless of the driver's offset.
• timezone=True columns are returned unchanged, keeping their awareness.
• None and string inputs are handled exactly as before.

Tests

tests/unit/test_databend_dialect.py — server-less, calls result_processor directly:

• tz-aware UTC input → naive output for a timezone=False column;
• a non-UTC (+02:00) input is normalised to UTC, not merely truncated;
• naive passes through unchanged, None is preserved, strings still parse;
• a timezone=True column keeps its tzinfo.