Fix missing zone history and landscape data on multi-device accounts

[sebr]

Summary

Fixes #393. On accounts with more than one sprinkler_timer, zone history for every device after the first showed unknown starting in 4.0.0.

Root cause: _last_poll_device_histories and _last_poll_landscapes in pybhyve/client.py were single scalar timestamps shared across all devices. The 4.0.0 coordinator fetches history/landscapes for each sprinkler sequentially on every tick — the first device updates the stamp, subsequent devices bail out at the rate-limit check milliseconds later, and their entries are never populated in _device_histories. get_device_history() then returns None → coordinator converts to [] → zone history sensor renders unknown.

Landscapes had the same shared-timestamp bug plus self._landscapes was a single list overwritten on each per-device fetch, so only the last device's landscape data survived regardless.

Fix: key both the state and the timestamps by device_id.

Test plan

• New tests/test_client.py covers:
  • Back-to-back history fetches for different devices each hit the API (regression for No longer getting zone history on HT25G2-0001 device since 4.0.0 update #393)
  • Same-device history fetch is rate-limited within the poll period
  • force_update=True bypasses the rate limit
  • Landscape fetches for different devices are both served
  • A zone lookup for one device does not fall through to another device's landscape list
  • Same-device landscape fetch is rate-limited
• Full suite: 148 passed, 5 skipped
• ruff check / ruff format --check clean