[Test] Add code coverage reporting and CI enforcement for unit tests

[oussamahansal]

Overview

We currently have no visibility into code coverage metrics. We need to add coverage to our test suite and enforce a
minimum baseline in CI to prevent regressions.

Acceptance Criteria

• pytest runs with coverage enabled by default

• CI fails if coverage drops below the agreed baseline

• Per-module coverage % is visible in CI logs

• Coverage report artifact is available on every CI run