Fix pin ergonomics pretty test for direct pin_v2 rule

[P8L1]

Summary

Fix the stage-2 pretty test failure in tests/pretty/pin-ergonomics.rs after adding the direct &pin / #[pin_v2] rule.

The test directly pins Foo with &pin mut Foo and &pin const Foo, but Foo was still a plain ADT. Since direct pinning of ADTs now requires #[pin_v2], the pretty-printed source no longer typechecked.

This PR marks Foo as #[pin_v2] so the pretty test remains valid while preserving the new diagnostic behavior.

Tests

Passed:

./x test tests/pretty/pin-ergonomics.rs --stage 2 --test-args --force-rerun
./x test tests/pretty/pin-ergonomics-hir.rs --stage 2 --test-args --force-rerun
./x test tests/pretty --stage 2 --test-args pin-ergonomics --force-rerun
./x test tests/ui/pin-ergonomics --stage 2 --test-args --force-rerun
./x test --stage 2 --skip tidy --skip compiler --skip src
git diff --check
./x fmt --check