Add pytest unit tests for opensensemap_api

[Copilot]

No tests existed for the opensensemap_api package. This adds a full pytest suite using unittest.mock (no external HTTP mocking library required).

Changes

• pyproject.toml — adds [project.optional-dependencies] test (pytest>=7, pytest-asyncio>=0.21) and configures asyncio_mode = "auto"
• tests/conftest.py — shared mock data: a full MOCK_STATION_DATA response with all sensor types, and a MOCK_STATION_DATA_MINIMAL for edge cases
• tests/test_opensensemap.py — 41 tests across four classes:

Class	Coverage
TestInitialization	base_url construction, empty initial data
TestProperties	All metadata properties; None fallbacks for optional keys
TestSensorValues	All 13 sensor value properties; missing sensor → None
TestGetValue	Exact-title lookup, case-insensitivity, all _TITLES language aliases, empty/missing lastMeasurement
TestGetData	Successful fetch; TimeoutError, ClientError, socket.gaierror each raise OpenSenseMapConnectionError

pip install -e ".[test]"
pytest tests/