After a reconnect, _ready_for_realtime_input was never set back to True
because _create_initial_response (which sets the flag) is only called on
initial connection. This caused all audio/video/text to be silently
dropped after reconnecting, making the bot appear to hang.
Set the flag in _handle_session_ready when we detect a reconnect, either
via session_resumption_handle (server restores state) or via existing
context (rare case where connection drops before first resumption handle).
