Return NotRunning consistently for live-node APIs #1141

API methods that require a running node do not consistently reject calls after the node has stopped. For example, OnchainPayment::bump_fee_rbf can reach wallet mutation and signing after Node::stop(), whereas the on-chain send methods return Error::NotRunning. This is a correctness issue in the API lifecycle contract.

Review all public API paths that require a live node and ensure they return Error::NotRunning when it is stopped, before performing side effects. Add missing checks consistently, including RBF fee bumps, and regression coverage for calls after shutdown and normal operation after restart. Preserve methods that intentionally support offline use. We should also address the TOCTOU race where a send passes its running-state check, shutdown completes, and the send then continues to mutate the wallet or queue a transaction.