/memory stats and /memory diagnose fall back to a generic
'Memory <action> is not available for the <backend.id> backend.'
message whenever the active backend's stats/diagnose hook is
undefined. For every real backend (hindsight, mnemopi, local) this
reads fine, but the off backend isn't a backend a user picked among
several stats-capable options - it's the no-op state memory falls
back to by default - so the same template renders as 'Memory stats
is not available for the off backend.', which reads as an odd,
almost self-contradictory warning.
Special-case backend.id === "off" with wording that matches the
phrasing offBackend.status() already uses elsewhere ('Memory backend
is off.'), and add a unit test covering both the off-backend wording
and the unchanged generic fallback for a real backend (local) that
simply has no stats hook.
The regression test only asserts on a mocked showWarning call and
never renders Markdown, so it doesn't need a real theme instance;
drop the global dark-theme setup/teardown to avoid leaving the
process-wide theme singleton mutated for later suites in the same
Bun process.
Kenneth Hoff