Creates an unsigned Avalanche C-Chain atomic export transaction (ExportTx) that exports AVAX from a custodian
C-Chain address (0x...) to a destination P-Chain address (P-...). This is step 1 of the C-to-P transfer flow.

The custodian does not send a nonce. CDAO reads the C-Chain account nonce from the node, estimates the atomic
export fee, builds the unsigned EVM atomic transaction, and returns unsignedTxHash, unsignedTxSerialized, and
transferSummary for custodian review and offline signing.

After signing, call /avalanche/transactions/prepare, then /avalanche/transactions/submit to dry-run or
broadcast the export through the C-Chain atomic API. A successful export does not make funds available on the
P-Chain yet; it creates an atomic UTXO that must be imported with /avalanche/transactions/import-c-to-p.