Address review: add clarifying comments for returncode and sig normalization

Agent-Logs-Url: https://github.com/codeSamuraii/pyfuse/sessions/50af12cc-7049-4c63-b673-b2a63099b2d2

Co-authored-by: codeSamuraii <17270548+codeSamuraii@users.noreply.github.com>

Author: Copilot

pyfuse/core/task.py

@@ -185,5 +185,5 @@ def from_json(
             timeout=data.get("timeout"),
             retries=data.get("retries", 0),
             retry_delay=data.get("retry_delay", 1.0),
-            signature=sig or None,
+            signature=sig or None,  # normalise empty string to None
         )

pyfuse/worker/deps.py

@@ -96,6 +96,7 @@ async def _pip_install(package: str, extra_args: list[str]) -> tuple[int, str, s
         stderr=asyncio.subprocess.PIPE,
     )
     stdout_bytes, stderr_bytes = await proc.communicate()
+    # returncode is None if the process hasn't terminated; treat as failure
     returncode = proc.returncode if proc.returncode is not None else 1
     return (
         returncode,