Broadcast a signed Babylon (Cosmos) transaction containing MsgCreateBTCDelegation.

This is step 4 of the BTC staking flow:

1. POST transactions/prepare/pre-delegate — Server prepares PSBTs and POP message
2. Client signs the PSBTs and POP message with their BTC key
3. POST transactions/prepare/delegate — Server assembles the MsgCreateBTCDelegation
4. POST transactions/broadcast/babylon — Client signs and broadcasts the Babylon transaction (you are here)
5. GET stakes/:txId/status — Client polls until delegation reaches VERIFIED or ACTIVE state
6. POST transactions/broadcast/bitcoin — Client broadcasts the signed Bitcoin staking transaction

Important: After this step succeeds, poll GET stakes/:txId/status using the bitcoinStakingTxId from step 1 until the delegation reaches VERIFIED or ACTIVE before broadcasting the Bitcoin transaction in step 6.