Fixes#1259
## Problem
The `CommandHandle.wait()` method fires `onStdout`, `onStderr`, and
`onPty` callbacks without `await`, so async callbacks run as
fire-and-forget microtasks. If a callback performs I/O (e.g. writing to
a file, sending over network), `wait()` can resolve before the callback
finishes, leading to lost data or race conditions.
## Fix
Add `await` before each optional-chain callback invocation:
```diff
-this.onStdout?.(stdout)
+await this.onStdout?.(stdout)
```
This is fully backwards compatible — `await`-ing a sync function's
return value is a no-op.
## Tests
3 parameterized test cases (`stdout`, `stderr`, `pty`) verify that
`wait()` does not resolve until an async callback's promise settles.
_This fix was developed with AI assistance and reviewed by a human._
Yizuki_Ame
ad377962dc