DateOnly/DateTime coercion gaps on 'date' columns (breaks with Npgsql 10, and SqlClient the other way)

Sibling of DapperLib/DapperAOT#202 (fixed on the AOT side in DapperLib/DapperAOT#203); vanilla has the same gap, in both directions. Adjacent to but distinct from #2072.

Npgsql 10 changed reader.GetValue() for a date column to return DateOnly rather than DateTime. Probed against Dapper 2.1.72 with a date column holding 2021-01-01:

probe	Npgsql 9.0.2 (DateTime box)	Npgsql 10.0.2 (DateOnly box)	SqlClient (DateTime box)
Query<DateTime>	✅	❌ DataException (inner: DateOnly is not IConvertible)	✅
Query<DateTime?>	✅	❌ same	✅
POCO DateTime member	✅	❌ same	✅
POCO DateOnly member	❌ DataException (DateTime box)	✅	❌ same as Npgsql 9

So there is no DateOnly↔DateTime coercion in either direction on the read path: providers that box DateTime break DateOnly members, and Npgsql 10 (boxing DateOnly) breaks DateTime members and typed scalars. Each provider/version combination has a working direction and a broken one, which makes upgrading Npgsql a breaking change for any model that reads dates as DateTime.

The AOT-side fix was simply to accept either box shape in the conversion helper, keyed on the runtime value — DateOnly → ToDateTime(TimeOnly.MinValue) for a DateTime target, and the equivalent in reverse; the same treatment fits FlexibleConvert/the parse paths here.

Repro is a 30-line console app (fresh postgres:16 container + select '2021-01-01'::date); happy to PR the fix.