[Testing] Coverage visibility: absolute metrics, artifacts, per-package targets, and multiprocessing collection (v1.0.0)

[Bslabe123]

What would you like to be added:

Augment the existing coverage ratchet (scripts/check_coverage_regression.py) with absolute coverage metrics, CI artifacts, and a Codecov badge, and set per-package targets instead of a single global floor. Also fix coverage collection under multiprocessing.

Why is this needed:

The current ratchet enforces delta and regression but gives no absolute visibility, and it relies on one arbitrary global floor. Separately, coverage does not trace subprocesses without concurrency=multiprocessing / COVERAGE_PROCESS_START, so the multi-process request collector and other subprocess code read as uncovered today even when they are exercised, which distorts the numbers and hides real gaps.

Proposal:

1. Emit absolute coverage metrics and publish them as CI artifacts; add a Codecov badge.
2. Define per-package coverage targets rather than a single global floor.
3. Enable multiprocessing-aware coverage so subprocess code (e.g., the request collector) reports real numbers.

Part of the v1.0.0 testing strategy: design doc.

[Bslabe123]

Tracked under #519.