Assemble a MsgCreateBTCDelegation Babylon transaction from signed BTC items.

This is step 3 of the BTC staking flow:

1. POST transactions/prepare/pre-delegate — Server creates staking TX, slashing PSBTs, unbonding PSBTs, and a POP message
2. Client signs the PSBTs and POP message with their BTC key
3. POST transactions/prepare/delegate — Server assembles the MsgCreateBTCDelegation (you are here)
4. POST transactions/broadcast/babylon — Client signs and broadcasts the Babylon transaction
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

Inputs Required:

• Session state from step 1
• Signed slashing PSBT, signed unbonding slashing PSBT
• POP signature (base64)
• Signed staking PSBT
• Original staking parameters (address, public key, amount, timelock)

Returns:

• Unsigned Babylon (Cosmos) transaction in base64 (to be signed with Babylon key in step 4)
• Signed Bitcoin staking transaction in hex (to be broadcast in step 6, after delegation is verified in step 5)