Gate finding B08-1: when MATRIX_E2EE_MODE is optional/required but the
decryptor failed to set up at connect (degraded mode), the warning still
told the user to pip install the E2EE deps and set MATRIX_E2EE_MODE=optional
-- advice that is wrong (the mode is already on) and hides the real cause.
Keep the install hint only for mode=off; otherwise name the active mode and
point at the earlier Matrix E2EE startup warning. The warn-once test is
parametrized over off/optional to pin both messages.
1680245d7e