fix: SensorUpdate leaked mutable state across updates; add API test coverage

[bluetoothbot]

What

Make each SensorUpdate an immutable snapshot, and add behavioral test coverage for the previously-untested SensorData public API.

Why

SensorUpdate is declared frozen=True, but _finish_update() returned the live per-update dicts by reference. Those dicts are reused across calls — update() calls _events_updates.clear(), and update_sensor()/update_binary_sensor() reassign entries — so a previously-returned SensorUpdate would silently mutate when the next update() ran:

• its events got cleared out from under the caller (events are transient and .clear()-ed each cycle);
• its entity_values / descriptions were rewritten to the newer reading.

Any consumer that holds the prior update to diff against the next one (a common pattern) would read corrupted data. This came to light while adding coverage — the demonstrating tests fail against the pre-fix commit.

How

• _finish_update() now returns shallow dict(...) copies of each per-update mapping. The contained value/description objects are themselves frozen dataclasses, so a shallow copy is a complete point-in-time snapshot. Cost is a few small dict copies per update — negligible vs. correctness.
• New tests/test_data.py covers title, per-device info aggregation, primary_device_id, get_device_name (name→type fallback), supported(), binary sensors, device_id propagation, and two regression guards that pin the snapshot semantics.

Structured as two commits (failing tests → fix) so the before/after is reviewable.

Testing

PYTHONPATH=src python3.12 -m pytest tests/ → 25 passed (was 8). ruff check clean. The 3 snapshot tests fail on commit 1 and pass on commit 2.

---

Quality Report

Changes: 2 files changed, 275 insertions(+), 6 deletions(-)

Code scan: clean

Tests: passed (25 passed)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.37%. Comparing base (31bd826) to head (7f437a1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   94.37%   99.37%   +5.00%     
==========================================
  Files          12       12              
  Lines         480      480              
  Branches        5        5              
==========================================
+ Hits          453      477      +24     
+ Misses         26        2      -24     
  Partials        1        1              

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