RequireStarted sets the _started flag to True on __enter__, but never clears it on __exit__.

#### Environment details

- OS type and version: MacOS Sonoma 14.2
- Python version: 3.11
- pip version: 23.3.1
- `google-cloud-pubsublite` version: `1.9.0`

#### Steps to reproduce

1. Write a method that publishes a message to a topic within a context manager within the method.
2. Call that method.
3. Call that method again.

#### Code example

```
from google.cloud.pubsub_v1.types import BatchSettings
from google.cloud.pubsublite.types import CloudRegion, CloudZone, TopicPath, MessageMetadata
from google.cloud.pubsublite.cloudpubsub import AsyncPublisherClient

class AbstractPublisher:

def __init__(self, configuration):
self._configuration = configuration

location = CloudRegion(self._configuration.region)
if not self._configuration.regional:
location = CloudZone(location, self._configuration.zone_id)

# This topic obviously needs to exist on GCloud already: it needs to be defined beforehand.
self.topic_path = TopicPath(os.getenv("GOOGLE_CLOUD_PROJECT"), location, self._configuration.topic_id)

self._publisher = AsyncPublisherClient(
per_partition_batching_settings=BatchSettings(
# These are used for message flow control: messages being published within max_latency sec of each other
# are batched and sent in with a single request, up to max_messages messages and max_bytes bytes.
# This is to help limit rapid calls to the backend to once every max_latency in most circumstances.
max_bytes=self._configuration.max_bytes_num,
max_latency=self._configuration.max_latency_sec,
max_messages=self._configuration.max_messages_num
),
enable_idempotence=self._configuration.enable_idempotence
)

async def publish_bytes(self, message: bytes, key: str, **kwargs):
async with self._publisher as publisher:
message_id = await publisher.publish(self.topic_path, message,
publish_timestamp=str(datetime.utcnow()),
message_key=key,
**kwargs)

#### Stack trace

FailedPrecondition exception at RequireStarted:32 on google.cloud.pubsublite.internal.require_started.py
This on every call to publish_bytes after the first.

Take a look at RequireStarted class, please. Verify if the _started behavior. Thanks!

Kamen