fix: flush StreamedLogAsync tail when stop() cancels the task (#754)

## Summary

In `StreamedLogAsync` (`src/apify_client/_streamed_log.py`), shutdown is
driven by `stop()` cancelling the streaming task. `CancelledError`
unwinds out of the `async for` before the trailing
`_log_buffer_content(include_last_part=True)` runs, so whatever sits in
the buffer (the tail after the last 8601 marker) is silently discarded.

Wrapping the loop in `try/finally` guarantees the tail is flushed even
on cancel.

A regression test covers the bug.