Fix worker response reading when process has exited
In `SingleplexWorker.getResponse`, when `process.isAlive()` returned `false`, Bazel previously checked `recordingInputStream.available() > 0` before breaking out of the loop. On macOS/BSD, `available()` on a pipe returns `0` after the writer process has exited, even if unread bytes (`WorkResponse`) remain in the OS pipe buffer.
This caused `SingleplexWorker` to throw a false-positive `IOException("Worker process ... died while waiting for response")` on macOS when a worker voluntarily exited after completing a task (e.g., `--exit_after=1`).
By breaking out of the loop when `!process.isAlive()`, Bazel attempts to read and parse the response via `workerProtocol.getResponse()`. If the worker produced a response, it is successfully parsed. If the worker crashed without returning a response, `getResponse()` returns `null` or throws an exception, resulting in proper error handling in `WorkerSpawnRunner`.
PiperOrigin-RevId: 969859950
Change-Id: I357b1e29ed107cd20896f00d123bdc42891f52db
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