Creates an unsigned Avalanche P-Chain delegation transaction (AddPermissionlessDelegatorTx) to delegate AVAX to
a Primary Network validator. The custodian supplies the account, funding P-Chain wallet, delegation amount,
duration, optional reward address, and optionally a target nodeId.

If nodeId is omitted, CDAO selects an eligible validator from its managed validator pool automatically. If
nodeId is provided, CDAO validates that the validator is allowed by backend policy, active, has enough remaining
duration, and has enough delegation capacity. The final selected validator is always returned as
transactionSummary.nodeId for custodian verification before signing.

The response includes the canonical unsigned transaction bytes, the unsignedTxHash to sign, and a summary of
the resolved delegation period, reward address, estimated P-Chain fee, change amount, and available validator
capacity snapshot.

Use /avalanche/transactions/prepare to attach the custodian signature, then /avalanche/transactions/submit
to dry-run or broadcast the signed transaction to the P-Chain. Validator capacity and UTXO freshness are
preflight checks and must be revalidated again immediately before broadcast.