Format NpgsqlRange<T> bounds culture-invariantly in ToString and Parse - #6738

NpgsqlRange<T>.ToString() formatted bounds with the current culture, so under de-DE a numrange printed as [1,5,2,25), and a daterange printed as [01.07.2026,04.07.2026], which PostgreSQL reads as other dates. This is what EF Core's ToQueryString() and parameter logs show (details in the issue).

Bounds are now formatted with the invariant culture, and DateTime, DateTimeOffset, DateOnly and TimeOnly bounds use the round-trip format (O).

There was no reply on the issue about the date format, so these behavior changes are worth a look. I'm happy to narrow the PR if you'd rather keep any of them out:

• Date/time text changes under every culture. For example, under the invariant culture [07/01/2026,07/04/2026] becomes [2026-07-01,2026-07-04], and DateTime bounds keep their fraction digits. Anything that depends on the old text will see a difference.
• Parse now uses the invariant culture too. I changed it so ToString output round-trips under any culture, which goes beyond what the issue asks for. As a result, Parse no longer accepts culture-specific text such as [1,5,2,25) under de-DE.
• UTC DateTime bounds come back as local time. Round-tripping through ToString and Parse keeps the instant, but Kind changes from Utc to Local, because DateTimeConverter converts the Z value to local time.

Verification: RangeTests on net10.0 against PostgreSQL 18 (92 passed, 1 skipped), with this commit rebased on current main. I didn't run the rest of the suite. The new tests run under de-DE and cover decimal, DateTime, DateTimeOffset, DateOnly and TimeOnly.

Fixes #6730