Submit signed transactions as a Jito bundle for atomic execution. Used for partial-sell (tip + split + close) and multi-step operations.

Partial Sell Flow:

1. POST /positions/partial-sell → get splitTransaction + closeTransaction
2. Build a tip transaction (transfer to Jito tip account, use /bundle/tip for amount)
3. Sign all 3 transactions
4. Submit here as [tipTx, splitTx, closeTx]