Python SDK: expose consumer shutdown and offset drain timeout

### Description

The Rust high-level consumer exposes an asynchronous `shutdown()` method that drains pending offset work, performs final commits when enabled, leaves the consumer group, and stops its connection watcher.

The Python `IggyConsumer` currently has no corresponding method. Its `consumer_group()` builder also does not expose the Rust builder's `offset_drain_timeout`, which controls how long shutdown waits for background offset tasks.

This covers B22 and the currently applicable part of B23 in the Python SDK parity work tracked in #3893.

The `commit_failed_messages` setting listed in B23 is not present in the current Rust SDK and should not be added to the Python API.

### Affected area / component

Python SDK

### Proposed solution

- Add `IggyConsumer.shutdown()` as an asynchronous method that locks the wrapped Rust consumer, calls its `shutdown()`, and maps any error to `RuntimeError`.
- Preserve the Rust method's idempotent behavior.
- Add an optional `offset_drain_timeout: datetime.timedelta | None` argument to `IggyClient.consumer_group()`.
- When provided, convert it to `IggyDuration` and pass it to `IggyConsumerBuilder::offset_drain_timeout()`.
- When omitted, preserve the Rust builder default of five seconds.
- Regenerate `foreign/python/apache_iggy.pyi` and document the supported shutdown call order.
- Extend `foreign/python/tests/test_consumer_group.py` to verify:
- A consumer can be shut down after its consumption loop exits.
- Repeated shutdown calls succeed.
- A custom offset-drain timeout is accepted and propagated.
- Shutdown leaves a consumer group promptly so another member can receive its partitions.

Adding a replacement for the removed `commit_failed_messages` option is outside this issue.

### Alternatives considered

Continue relying only on the `asyncio.Event` accepted by `consume_messages()`. That event stops one consumption loop but does not expose the Rust consumer's resource and offset cleanup operation.

### Contribution

- [ ] I'm willing to submit a pull request to implement this feature

### Good first issue

- [x] I think this could be a good first issue for a new contributor

Start with the Python SDK's IggyConsumer and IggyClient.consumer_group() entry points, then compare the Rust high-level consumer shutdown() and offset_drain_timeout() behavior. Review foreign/python/tests/test_consumer_group.py and regenerate foreign/python/apache_iggy.pyi. Done means shutdown is idempotent, timeout propagation and prompt group departure are covered, and the supported shutdown call order is documented.

Written by the indexing model from the issue text.