fix: avoid PTY wait deadlock on wait errors

[branchseer]

Summary

• Fix the PTY wait deadlock by ensuring the child-monitor thread always sets the shared completion slot, including wait failures.
• Store wait failures as Arc<std::io::Error> in the shared OnceLock (Result<ExitStatus, Arc<std::io::Error>>) so error state is shareable without synthetic fallback status.
• Change ChildHandle::wait() to return anyhow::Result<ExitStatus> and propagate wait failures through callers.
• Update downstream helpers/callers (pty_terminal_test, milestone tests, and vite_task_bin e2e harness) to consume the new anyhow::Result return type.
• Keep PTY integration tests strict at #[timeout(5000)] and run them without a Windows serialization guard.

Stable Reproduction (before fix)

To deterministically reproduce the real deadlock path (without mocking child.wait() return values), I used a temporary Windows-only repro patch that swaps the internal WinChild process handle with a non-process handle (CreateEventW event handle). This makes child.wait() execute the real Windows wait/get-exit-code path and return Err.

With the old code path (if let Ok(status) = child.wait() { ... }), OnceLock was never set on error, and ChildHandle::wait() blocked forever.

Repro command:

cargo xtest --builder cargo-xwin --target x86_64-pc-windows-msvc -p pty_terminal --test terminal -- read_to_end_returns_exit_status_nonzero

Observed behavior under repro:

• timed out at ~15s with #[timeout(15000)]
• timed out again at ~60s with #[timeout(60000)]

That demonstrates timeout-independent deadlock behavior.

Why this fixes it

• Background monitor thread now always sets the shared wait result (Ok or Err).
• ChildHandle::wait() cannot block forever on an unset OnceLock.
• Wait failures are preserved and surfaced as real errors (no synthetic ExitStatus::with_exit_code(1)).

Validation

• cargo clippy -p pty_terminal --all-targets --all-features -- -D warnings
• cargo test -p pty_terminal -p pty_terminal_test
• cargo test -p vite_task_bin --test e2e_snapshots --no-run
• cargo xtest --builder cargo-xwin --target x86_64-pc-windows-msvc -p pty_terminal --test terminal
• cargo xtest --builder cargo-xwin --target aarch64-pc-windows-msvc -p pty_terminal --test terminal
• cargo xtest --builder cargo-xwin --target x86_64-pc-windows-msvc -p pty_terminal --test terminal repeated 40 times (all green, without serial guard)