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:

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.

Start at FlexibleConvert and the parse paths mentioned in the issue, then reproduce the behavior with the described 30-line console app using a date column. Trace conversions for both DateOnly and DateTime runtime values. Done means reads work in both directions for typed scalars and POCO members across the provider boxing shapes described.