[C8] Full test suite is Linux-only; macOS only smoke-tests `doctor`

[DsThakurRawat]

Labels: ci, testing, priority:medium
File: pytest.yml:12 (ubuntu-latest, py3.10/3.12 only), doctor-smoke.yml:42 (macOS runs only tether doctor)

Description. No unit/integration test runs on macOS; Windows is entirely absent.

Why it matters. Apple Silicon is a touted, supported target (README/pyproject + [onnx] path), yet Mac-specific breakage in serve/export/CLI (paths, ORT CPU EP, threading, Rich rendering) reaches Mac users unverified.

Tasks.

• Add macos-latest to the pytest.yml matrix (at least import/CLI/serve subset with [onnx,serve]).
• Explicitly document Windows as unsupported if that's the intent.

Dedup. Not covered.