Close coverage gap: deterministically cover stop_server() no-server branch (interface.py:285)

[tschm]

Problem

make test reports a single uncovered line: src/pycharting/api/interface.py:285 — the else: print("ⓘ No active server to stop") branch in stop_server(). Coverage sits at 99.80% (gate is 90%) solely because of this line.

A test for the branch already exists (tests/test_interface.py:168 test_stop_server_when_not_running), but it does not cover the line reliably.

Root cause

_active_server is a module-level global. The test calls stop_server() without resetting _active_server to None first. Under pytest-xdist parallelism, another test's running mock server leaks into the global, so the if _active_server and _active_server.is_running: branch is taken instead of the else. The test has no assertion on output, so it passes either way and the else branch goes uncovered.

Proposed fix

In tests/test_interface.py::test_stop_server_when_not_running, force the global to None before the call and assert on the message, e.g.:

def test_stop_server_when_not_running(self, monkeypatch, capsys):
    monkeypatch.setattr("pycharting.api.interface._active_server", None)
    stop_server()
    assert "No active server to stop" in capsys.readouterr().out

Acceptance criteria

• make test reports src/pycharting/api/interface.py at 100% (no missing line 285) across repeated parallel runs.
• The test asserts on the emitted "No active server to stop" message rather than merely not raising.

Subcategory: Test coverage & depth (9 → 10). Highest-leverage fix from the Rhiza quality assessment.