fix(moonrun): clear errno for macOS pid fallback

[peter-jerry-ye]

Why

moonbitlang/async#505 enabled process tests that exposed a macOS-only failure in the WASM async host. The macOS PID-handle path intentionally returns the invalid-handle sentinel to select the kqueue fallback, but unlike the native C implementation it left a stale errno behind. Process::from_pid then interpreted that stale errno as a new failure.

What changed

• Clear async-host errno after the macOS PID-handle fallback succeeds.
• Add a WASM integration regression that seeds errno before opening the PID handle and verifies the fallback clears it.

Why this is correct

The change restores parity with the native implementation, which returns the sentinel with errno set to zero. The sentinel still selects the existing kqueue path; only the stale error state is removed.

Scope

The runtime change is limited to the non-Linux Unix PID-handle fallback. Other platforms and process paths are unchanged.

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/moonrun/src/async_api/process.rs	31% smaller
crates/moonrun/tests/test_cases/test_async_host.in/main/main.mbt	Unsupported file format