Submits a signed Avalanche transaction returned by /avalanche/transactions/prepare. CDAO decodes the signed
transaction, verifies it matches the stored unsigned transaction and account context, reruns final freshness and
policy checks, and either dry-runs the submission or broadcasts it.

When dryRun is true, CDAO performs the same pre-submit checks but does not call any Avalanche issueTx method.
No network transaction is created and no real txHash should be returned. When dryRun is false, CDAO routes the
transaction to the correct chain API: C-Chain atomic export transactions go to the C-Chain atomic API, while
P-Chain stake, delegate, and import transactions go to the P-Chain API.

RPC acceptance only means the network accepted the transaction for processing. Use /avalanche/transactions/status
to track finality: C-Chain atomic export transactions become final when ACCEPTED; P-Chain transactions become
final when COMMITTED.