How to serialize a transaction from mempool and sign it into a bundle

i just test signBundle via flashbots, i feel confused. there is the code

```javascript

// the transaction is get from geth mempool
const transaction = {
"blockHash": null,
"blockNumber": null,
"from": "0x0160ea15d2125dd9c211f23169c4e4f4fb7957a8",
"gas": "0x4c4b40",
"gasPrice": "0x1ca10",
"hash": "0x696968e15c7174609adca25769c4e7c0e2c18d1eedbca8e4b71e3a0f59c52797",
"input": "0xc652885f0000000000000000000000009bd2aa38429c356be17c8c7bb07dab299200eeb84178ca8820c1a8dae966f1fdd132a2a874394233000000000000000000000054000000000000000000000000000000000000000000000000000000000000006000000000000000000000000000000000000000000000000000000000000000010000000000000000000000004178ca8820c1a8dae966f1fdd132a2a8743942330000000000000000000000000000000000000000000000000000000000000320",
"nonce": "0x163",
"to": "0x7ede79ed0d3acf3d4843c919975fb7bd97ecccba",
"transactionIndex": null,
"value": "0x0",
"type": "0x0",
"chainId": "0x5",
"v": "0x2e",
"r": "0xf4556687cb3df0f345ab903a2c9c46bb1a9fb413a0f498e8dba3b74253caa313",
"s": "0x281adb2e3f2b73c685aaaec10120b9ade46d6c2cd069d1106ae5093833d0bbf1"
};

const signedBundle = await flashbotsProvider.signBundle([
{
signer: transactionFromSigner.connect(provider),
transaction: {
...baseTransaction,
gasPrice: '',
data: '0x'
},
},
{
// there, i don't know how to serialize transaction from mempool
signedTransaction: ethers.utils.serializeTransaction(transaction)
}
]);
```

Start with the ethers.utils.serializeTransaction call and the signBundle usage shown in the issue, then compare them with the repository's bundle examples and serialization requirements. Done means there is a documented, reproducible answer for handling the supplied mempool transaction, including the required fields and signing constraints.

Written by the indexing model from the issue text.