Fix order-dependent test via global-server-state reset

[tschm]

Subcategory: Test pass rate & test-design quality — 6 → 10

Problem: test_status_when_no_server (tests/test_interface.py:177) is order-dependent. Under xdist a server started by a sibling test on the same worker leaks into it because _active_server is a module-global (src/pycharting/api/interface.py:32) that is never reset between tests.

Evidence (from make test):

[gw13] tests/test_interface.py:181: AssertionError
assert True is False   # status["running"] expected False, got True

Change: Add an autouse fixture that resets _active_server / stops any running server between tests (or derive status from live state), so worker ordering can't leak a running server into the no-server test.

Done when: make test is green across repeated -n auto runs.

Context: PR #15