test: add package coverage reporting workflow

[AlwinXue]

Hi mentors! 👋

This PR addresses Issue #30 by adding automated package-level coverage reporting to the CI pipeline.

The Problem:
The repository currently has a release workflow for publishing the Python package, but it does not provide a CI signal for test coverage. That makes it harder to track whether the package test suite is expanding in a meaningful way as reliability work continues.

The Solution:
Package Test Extras: Added a test optional dependency group in package/pyproject.toml so the CI workflow can install package test dependencies in a standard way.
Coverage Workflow: Added a new GitHub Actions workflow that installs the package in editable mode with test dependencies, runs the package test suite with pytest-cov, prints terminal coverage, and uploads coverage.xml and pytest.xml as build artifacts.
Focused Scope: This first version targets package-level coverage only, which keeps the workflow stable and avoids unrelated backend system-dependency issues.

Validation:
Ran python -m pip install -e "./package[test]" and python -m pytest package -q --cov=pyaslreport --cov-report=term-missing --cov-report=xml:package/coverage.xml locally; the package suite passed and produced a coverage report.

Note:
This PR intentionally builds on the isolated package test structure introduced in my earlier testing PR so coverage reporting runs against a cleaner package test layout.

Fixes #30