Add tests, CI workflows, and homeassistant mock for task_tracker

[Copilot]

No tests existed for the integration. This adds comprehensive unit tests for all non-boilerplate code, a minimal homeassistant mock to avoid the full HA dependency, and pylint/CI GitHub workflows.

Test coverage

• test/test_validate_options.py — 20 tests for options_flow.validate_options(): all default values, boundary conditions (interval ≥ 1, mdi: prefix), and passthrough of valid input
• test/test_sensor.py — 30 tests for TaskTrackerSensor: __init__, async_update (due-date arithmetic for all interval types), _filter_state_changes, async_sync_todo_list (all 4 add/update/remove/noop branches), async_mark_as_done, async_set_last_done_date
• test/test_config_flow.py — TaskTrackerConfigFlow.async_step_user: form shown on no input, entry created with correct data/options, options flow delegation
• test/test_init.py — async_migrate_entry (v1.1→v1.2 migration, version guards) and get_sensor
• test/test_button.py — button.get_sensor: single sensor found, zero sensors, multiple sensors

Homeassistant mock

test/homeassistant_mock/ is a minimal stub of all homeassistant sub-packages imported by the integration (components, config_entries, core, exceptions, helpers, util). Tests inject it via sys.path — same pattern used in gensyn/ssh_command#2.

Infrastructure

• requirements.txt — python-dateutil and voluptuous (the only non-stdlib, non-HA runtime deps)
• .github/workflows/test.yml — Python 3.13, installs requirements.txt, runs python -m unittest discover -s test
• .github/workflows/pylint.yml — Python 3.13, --fail-under=9, --ignore-paths=^test/.*$
• run_tests.sh — local convenience wrapper using coverage
• .gitignore — excludes __pycache__, .pyc, coverage artifacts

Original prompt

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This section details on the original issue you should resolve

<issue_title>No tests in repository</issue_title>
<issue_description>Create tests for all non-boilerplate code use https://github.com/gensyn/HomeAssistantPlugin as a template for tests and also add the pylint and test GitHub workflows from that repo to this. Do not introduce a new requirements_unittest.txt but use the existing requirements.txt. Mock everything relating to homeassistant as needed.

Also have a look at this very similar PR: gensyn/ssh_command#2</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes No tests in repository #4

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