**PR Title:** "Add wallet_db attribute to MempoolNode and initialize in constructor"

[Sophie-Ag00001]

## Summary
This PR adds a `wallet_db` attribute to the `MempoolNode` struct and initializes it in the `MempoolNode::new()` function using the provided `WalletDb` initialization code. The `WalletDb` is created with configuration parameters and extra data already available in the `MempoolNode::new()` context.

## Motivation
The addition of the `wallet_db` attribute is necessary to integrate wallet database functionality into the `MempoolNode` struct. This change ensures that the mempool node has access to wallet-related data and operations, which is essential for supporting wallet features within the mempool.

## Context
The `WalletDb` initialization requires specific configuration parameters (`mempool_db_mode`, `passphrase`, and `custom_wallet_spec`) and extra data (`wallet_db`). These are already accessible in the `MempoolNode::new()` function, making it straightforward to initialize the `wallet_db` attribute during node creation.

## Testing
To test the changes:
1. Ensure that the `MempoolNode::new()` function correctly initializes the `wallet_db` attribute.
2. Verify that the `wallet_db` is properly integrated into the `MempoolNode` struct and can be accessed as needed.
3. Run existing tests to confirm that no existing functionality is broken by this change.

## Known Issues
- None at this time. This change is limited to adding the `wallet_db` attribute and its initialization, with no other code modifications.