test: raise timeout for presigned URL fetches in integration tests (#757)

## Summary

The 5s HTTP timeout on presigned-URL fetches in integration tests was
too tight for the external Apify platform and occasionally tripped,
causing flaky failures (e.g. [this job
run](https://github.com/apify/apify-client-python/actions/runs/24832416128/job/72683975229?pr=738)).
Bumped the timeout to 30s in all four affected tests (`test_dataset.py`,
`test_key_value_store.py`) to absorb transient slowness while still
failing fast on a truly broken endpoint.