policy-engine: FFI test coverage dropped from 15 Rust tests to 1 after the retarget

core/tests/ffi_roundtrip.rs and ffi_default_dispatchers.rs (15 tests) were removed with the embedded engine; only assert_ffi_uses_opa in sdk/rust/tests/upstream_compatibility.rs remains. The .NET harness covers part of the surface. Restore roundtrip and default-dispatcher coverage against the pinned crate.

Found in the group review of the policy-engine retarget (#3939) and deferred from the follow-up (#3940).

Start by reviewing the retarget changes in #3939 and the pinned crate configuration, then use the removed core/tests/ffi_roundtrip.rs and ffi_default_dispatchers.rs paths to reconstruct the intended coverage. Run the Rust test suite alongside sdk/rust/tests/upstream_compatibility.rs and confirm the 15 roundtrip and default-dispatcher tests are restored against the pinned crate.

Written by the indexing model from the issue text.