PR #47 follow-up for @Larslllllll: run pytest in CI and verify coverage

[netanelcyber]

@Larslllllll — CI follow-up to merged PR #47.

Context

The existing Windows/Ubuntu workflow compiles the package and runs a dry-run, but it does not execute the pytest suite added by PR #47. Broader CI/coverage tracking remains in #31.

Minimum changes

• Install pytest in the existing CI matrix.
• Run python -m pytest -q tests/test_core.py on Windows and Ubuntu.
• Make CI fail when the core tests fail.
• After the test-validity fixes in PR #47 follow-up for @Larslllllll: make core tests behavior-valid #50 land, run the corrected suite in CI.
• Add coverage measurement/reporting and record the actual core-function coverage before claiming the 70%/80% targets.
• Correct the coverage description so it does not claim absent VRFY negative/error cases or POP3/IMAP tests.

Acceptance

CI must exercise the tests rather than only compiling/dry-running, and reported coverage must be measured rather than inferred from test count.

Related: #31, #50, PR #47.