[Enhanced-parts] No error in the analyzer when parts have different language versions

[sgrekhov]

There is no error in the analyzer in the following cases.

1. main.dart has // @dart=3.11
part1.dart part of main.dart has // @dart=3.11
part2.dart part of part1.dart has // @dart=3.10

2. main.dart has // @dart=3.10
part1.dart part of main.dart has // @dart=3.10
part2.dart part of part1.dart has // @dart=3.11

3. main.dart has // @dart=3.11
part1.dart part of main.dart has // @dart=3.11
part2.dart part of part1.dart has no language marker

4. main.dart has no language marker
   part1.dart part of main.dart has no language marker
   part2.dart part of part1.dart has // @dart=3.11

There should be a compile-time error in all of the cases above (and this error is reported by CFE).

Dart SDK version: 3.11.0-edge.47f4ff25b7639c765156df145b131373e24f6652 (main) (Sun Oct 26 15:14:41 2025 -0700) on "linux_x64"

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/458083