Add Xiaomi Smart Pet Fountain 2 support

[JanOstrowka]

Summary

• add first-class python-miio support for Xiaomi Smart Pet Fountain 2 (xiaomi.pet_waterer.70m2)
• expose a dedicated MIOT device class, typed status enums, and control methods
• include tests for the main controls and missing-value handling

Why

Home Assistant core review requested that the pet fountain implementation live in the library used by the integration instead of being carried as a local helper in xiaomi_miio.

Included

• XiaomiPetFountain device class
• status model for fountain status, water mode, charging state, DND times, battery, filter metrics, and fault state
• control methods for mode, water interval, child lock, do not disturb, DND start/end, and filter reset
• export from miio.__init__
• README supported-device entry

Related

• Add Xiaomi Pet Fountain 2 support to xiaomi_miio home-assistant/core#166815

Validation

• python3 -m py_compile on the new pet fountain package and tests
• the equivalent entity model and control flow were exercised against a real device from the Home Assistant side while iterating on the integration support

I was not able to run the full pytest suite in this environment because pytest is not installed locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.48%. Comparing base (c5cc0f2) to head (92c310b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2069      +/-   ##
==========================================
+ Coverage   82.26%   82.48%   +0.21%     
==========================================
  Files         197      201       +4     
  Lines       19145    19381     +236     
  Branches     1052     1060       +8     
==========================================
+ Hits        15750    15986     +236     
  Misses       3218     3218              
  Partials      177      177              

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[JanOstrowka]

I split the Read the Docs cleanup into a dedicated follow-up PR: #2070.

That PR fixes the warning sources causing the RTD build to fail on Python 3.9 / Sphinx 7.4.7, and I verified it locally with:

/tmp/python-miio-rtd-rtd/bin/sphinx-build -b html -W docs /tmp/python-miio-rtd-rtd74-html

So the remaining RTD failure on this feature PR should be addressable independently of the pet fountain implementation.