Add pytest test suite with 96% coverage and GitHub Actions CI

[geofffranks]

Summary

• Replaces the broken tests/test_utec.py (wrong imports, references to non-existent methods like _api_call/_discover and UtecOAuth2 which lives in the HA integration, not this library) with a working pytest suite across 10 new test files covering API transport, device classes, auth, exceptions, and constants.
• Adds pytest-asyncio + coverage configuration in pyproject.toml with a fail_under = 90 gate, plus a pinned requirements-test.txt and a rewritten tests/conftest.py providing shared fixtures (mock_api, discovery_dict, state_payload, _FakeAuth).
• Adds a GitHub Actions workflow (.github/workflows/test.yml) that runs the suite with coverage on push to main and on all pull requests, across a Python 3.11 / 3.12 matrix.
• Current result: 126 tests passing in ~0.8s, 96% total line coverage. Per-module: api.py/auth.py/const.py/exceptions.py 100%, light.py 99%, lock.py 98%, device.py 96%, switch.py 94%, device_const.py 87%.

Test Plan

• pip install -e . -r requirements-test.txt
• pytest tests/ — expect 126 passed, 0 failed
• pytest tests/ --cov=utec_py --cov-report=term-missing — expect ≥90% overall, all required modules ≥90%
• Confirm the new CI workflow triggers and goes green on this PR
• Spot-check a few tests for structure (e.g. tests/test_api.py for HTTP transport via aioresponses, tests/test_lock.py for device state assertions)

Notes for Maintainers

• Zero src/ changes. Several tests adapt to observed source behavior rather than "correcting" it — for example LockState/DoorState enum values are title case, BaseDevice.update wraps errors as DeviceError, battery_level maps integer keys {1→10, 2→30, 3→50, 4→70, 5→100}, and DeviceError inherits directly from Exception rather than UHomeError. These are documented inline where relevant.
• docs/superpowers/plans/ contains the implementation plan and source audit used to produce this branch. They are process artifacts — feel free to squash them out of the merge if you prefer a cleaner history.
• Known non-blocking simplification: tests/test_api.py defines its own _FakeAuth subclass that duplicates the one in tests/conftest.py. Both work; consolidation can be done in a follow-up if desired.

[geofffranks]

Sorry for the terseness in the PR notes, and plethora of commits, Claude got a bit much there.

Essentially - Adds a ton of test coverage to bring the overall coverage up to 96%. This also adds a github action for auto-running the tests for PRs and for commits to the main branch. As such didn't want to push/merge directly without checking with you first on this one @LF2b2w.

[geofffranks]

Ah, also it looks like branch protection + reviewer requirements effectively prevent me from merging/pushing on my own anyway. If that was intended and i misinterpreted, no worried, just mentioning in case.

[geofffranks]

@LF2b2w bumping this PR as well