feat: export base description classes from package root

[bluetoothbot]

What: Export the base description classes (BaseDescription, BaseSensorDescription, BaseBinarySensorDescription) from the package root.

Why: SensorLibrary exposes BaseSensorDescription instances as its public API (e.g. SensorLibrary.BATTERY__PERCENTAGE), but the class itself was not importable from sensor_state_data. Consumers iterating the library could not annotate or isinstance-check those values against any exported type — they are also not instances of the already-exported SensorDescription.

How: Add the three base classes to the __init__ imports and __all__. Purely additive; no behavior change, nothing removed or renamed.

Testing: New tests/test_public_api.py pins the contract — base classes importable from root, SensorLibrary entries are BaseSensorDescription instances, and every __all__ name resolves. Full suite passes (11 in the touched set), ruff clean.

---

Quality Report

Changes: 2 files changed, 38 insertions(+), 1 deletion(-)

Code scan: clean

Tests: passed (11 passed)

Branch hygiene: clean

Generated by Kōan

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.37%. Comparing base (31bd826) to head (a45214b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files          12       12           
  Lines         480      480           
  Branches        5        5           
=======================================
  Hits          453      453           
  Misses         26       26           
  Partials        1        1           

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