test_utils: new module cancel_safe (#936)

Summary:
Pull Request resolved: #936

introduce `test_utils::cancel_safe` module with helpers for verifying future cancellation safety. provides sync (`assert_cancel_safe`) and async (`assert_cancel_safe_async`) variants that systematically test cancellation at every `Poll::Pending` boundary by driving a future to completion once to establish ground truth, then for each possible cancellation point `k` polling a fresh future `k` times, dropping it to simulate cancellation, and verifying a subsequent clean run still produces expected results. the async variant supports external progress drivers via `on_pending callbacks` for advancing mocked time or other runtime state. includes comprehensive documentation on cancel-safety semantics and examples covering pure state machines and timer-based futures. the intent is to provide a foundation for systematically validating that hyperactor's async primitives maintain their cancellation safety guarantees under arbitrary executor drop behavior. co-authored with mariusae

Reviewed By: mariusae

Differential Revision: D80626978

fbshipit-source-id: fb7405fcc55f018b0e6df89bec98d332ae083e0f

Author: shayne-fletcher

hyperactor/Cargo.toml

@@ -72,6 +72,7 @@ unicode-ident = "1.0.12"
 
 [dev-dependencies]
 maplit = "1.0"
+proptest = "1.5"
 timed_test = { version = "0.0.0", path = "../timed_test" }
 tracing-subscriber = { version = "0.3.19", features = ["chrono", "env-filter", "json", "local-time", "parking_lot", "registry"] }
 tracing-test = { version = "0.2.3", features = ["no-env-filter"] }