Serialize FFI tests that share panic state

[Copilot]

The test Actions job intermittently failed when an FFI alias-compilation test ran concurrently with the panic-guard test. The latter temporarily sets a process-global poison flag observed by guarded FFI calls.

• FFI test isolation
  • Run bindings/ffi tests with a single test thread.
  • Prevent the intentional poison-state test from contaminating unrelated FFI tests.

test_args.push(OsString::from("--"));
test_args.push(OsString::from("--test-threads=1"));

[Copilot]

Pull request overview

This PR makes the bindings/ffi test suite run single-threaded to avoid intermittent failures caused by tests interacting through a process-global panic/poison state during concurrent execution.

Changes:

• Pass -- --test-threads=1 to cargo test for bindings/ffi to serialize its Rust test harness.
• Improve isolation of the intentional poison-state test so it doesn’t contaminate unrelated FFI tests running in the same crate test run.

Suppressed comments (1)

xtask/src/tasks/bindings/ffi.rs:169

• The step label string no longer matches the actual invocation now that the test harness args include -- --test-threads=1. This can make CI logs misleading when diagnosing failures; update the label to reflect the full command.

        run_cargo_step(
            &ffi_dir,
            "cargo test --features contention_checks (bindings/ffi)",
            test_args,

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