The e2e docs claimed `e2e`-marked tests skip when no proxy answers the
liveness probe, but the harness has always hard-failed: conftest.py's
pytest_runtest_setup calls pytest.fail, its module docstring states
"hard failures only ... never skip", and logging/conftest.py forbids
skipping outright. Align the docs to the code so the single most
important contract reads the same everywhere; a dead proxy turns a run
red instead of being silently skipped and mistaken for a pass. The
per-suite conftest docstrings that described the shared hook as a
"proxy liveness skip" are corrected to "liveness gate" for the same
reason.
Also scope the no-unit-tests hard rule to what it means: never
substitute a unit test for e2e feature coverage, while explicitly
allowing tests that cover the harness itself (e.g.
coverage_registry/test_collector.py), which carry no e2e marker and
run whether or not a proxy is up.
No product code and no harness logic changed.
Resolves LIT-4554