Producer.poll(timeout) blocks for full timeout after delivery callback fires (regression in 2.13.0)

# Description

`Producer.poll(timeout)` no longer returns early after processing delivery callbacks when timeout > 200ms. Instead it blocks for the entire timeout duration, regardless of whether events were served.

This is a regression introduced in 2.13.0 by the "wakeable poll" refactor (PR #2126 ). The original single-call implementation of `rd_kafka_poll()` returned as soon as events were processed. The new chunked loop replicates this correctly for `Consumer.poll()` but not for `Producer.poll()`.

The producer loop accumulates `chunk_result` but never breaks when `chunk_result > 0`, causing it to exhaust all remaining chunks even after the delivery callback has fired.

# Step to reproduce
```python
import time
from confluent_kafka import Producer, __version__

KAFKA_BOOTSTRAP = "localhost:9092"
POLL_TIMEOUT = 5.0

print(f"confluent-kafka version: {__version__}")

producer = Producer({"bootstrap.servers": KAFKA_BOOTSTRAP})

def on_delivery(err, msg):
print(f" [{time.time():.3f}] on_delivery fired — err={err}, topic={msg.topic()}")

print(f"[{time.time():.3f}] produce()")
producer.produce("test-topic", value=b"hello", on_delivery=on_delivery)

print(f"[{time.time():.3f}] poll({POLL_TIMEOUT}) start")
t0 = time.time()
producer.poll(POLL_TIMEOUT)
elapsed = time.time() - t0
print(f"[{time.time():.3f}] poll() returned after {elapsed:.3f}s")
```

## Output in 2.12.2

```python
confluent-kafka version: 2.12.2
[1733305268.206] produce()
[1733305268.206] poll(5.0) start
[1733305268.532] on_delivery fired — err=None, topic=test-topic
[1733305268.532] poll() returned after 0.325s
```

## Output in 2.13.2

```python
confluent-kafka version: 2.13.2
[1733305281.264] produce()
[1733305281.265] poll(5.0) start
[1733305281.276] on_delivery fired — err=None, topic=test-topic
[1733305286.078] poll() returned after 4.813s
```

# Potential Cause

In `Producer_poll0` (`src/confluent_kafka/src/Producer.c`), the wakeable loop does not break when `chunk_result > 0`

# Potential fix

Add an early exit in `Producer_poll0` after events are served, matching the original `rd_kafka_poll()` single-call behaviour and the consumer implementation:

```python
int chunk_result = rd_kafka_poll(self->rk, chunk_timeout_ms);
if (chunk_result < 0) { r = chunk_result; break; }

r += chunk_result;
if (chunk_result > 0) break; // ← return early once events are served
```

The new chunked implementation was intended to preserve this early-exit behaviour while adding signal interruptibility. It does so correctly for `Consumer.poll()` (`if (rkm) break;`) but the equivalent guard was omitted in `Producer_poll0`, making `poll(5.0)` in 2.13.x strictly worse than `poll(5.0)` in 2.12.x when the broker is fast.

# Environment

- confluent-kafka-python: 2.13.0, 2.13.1, 2.13.2
- Python: 3.11+
- OS: Linux

---

Thanks to the maintainers for the great work on confluent kafka. Happy to open a PR with the proposed fix if the issue is
confirmed.