fix: Handle window closes correctly in reducestreamer

[BulkBeing]

Fixes: #328

When I tried to reproduce the issue, the reducestream UDF was crashing after running for sometime.

Reduce UDF:

class Counter(ReduceStreamer):
    async def handler(
        self,
        keys: list[str],
        datums: AsyncIterable[ReduceDatum],
        output: NonBlockingIterator,
        md: Metadata,
    ):
        logger.info(
            "Window opened: start=%s end=%s keys=%s",
            md.interval_window.start,
            md.interval_window.end,
            keys,
        )
        count = 0
        async for datum in datums:
            count += 1
            logger.info(
                "datum #%d: event_time=%s, watermark=%s",
                count, datum.event_time, datum.watermark,
            )
        logger.info("Window closed, total count=%d", count)
        await output.put(ReduceMessage(str(count).encode(), keys=keys))

When the platform (numa) closes sender end of the bidirectional gRPC stream between sliding window intervals, the in-flight await queue.get() in the consumer async iterator gets cancelled, raising asyncio.CancelledError. This is normal gRPC lifecycle — it's how the framework signals "this stream is done."

But ReduceFn had a blanket except BaseException that caught it. CancelledError is a subclass of BaseException (not Exception), so it was treated as a UDF error.

The caught exception was passed to handle_async_error → exit_on_error → psutil.Process.kill(), which killed the entire Python process. Since a reduce streamer has many concurrent windows/streams active at once, killing the process took down all of them.

After the fix:

Also verified UDF exception are reaching UI

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.10%. Comparing base (4d33841) to head (698bda3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...numaflow/reducestreamer/servicer/async_servicer.py	79.41%	2 Missing and 5 partials ⚠️
...numaflow/pynumaflow/reducestreamer/async_server.py	66.66%	3 Missing ⚠️
...pynumaflow/reducestreamer/servicer/task_manager.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
- Coverage   94.20%   94.10%   -0.11%     
==========================================
  Files          66       66              
  Lines        3091     3137      +46     
  Branches      162      168       +6     
==========================================
+ Hits         2912     2952      +40     
- Misses        149      151       +2     
- Partials       30       34       +4     

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[vigith]

https://github.com/numaproj/numaflow/blob/b4a2ece9a992aba651c70f2253e6b81ef46b4218/rust/numaflow-core/src/reduce/reducer/aligned/reducer.rs#L348-L351

[vigith]

remove this?