NetworkController: Standardize chainId Handling in Infura and Fetch Middleware

Description:
In the create-network-client.ts file, the createInfuraMiddleware and createFetchMiddleware functions currently handle chainId differently. This inconsistency could lead to potential issues or confusion when integrating with different blockchain networks.

Suggested Changes:

Align the handling of chainId in both middleware functions to use a uniform pattern for scaffolding out eth_chainId.
Ensure that both middleware functions process chainId in a similar manner, enhancing consistency and predictability in network interactions.
References:

[createInfuraNetworkMiddleware Function](https://github.com/MetaMask/core/blob/main/packages/network-controller/src/create-network-client.ts#L120C30-L120C30) - Current implementation of chainId handling.
[createCustomNetworkMiddleware Function](https://github.com/MetaMask/core/blob/main/packages/network-controller/src/create-network-client.ts#Ly) - Different approach to chainId handling.
Expected Outcome:
By standardizing the chainId handling across both middleware functions, we can achieve a more consistent and reliable network management within the MetaMask infrastructure.

Start in packages/network-controller/src/create-network-client.ts and compare chainId handling in createInfuraMiddleware with createFetchMiddleware, using the referenced implementations as the entry point. Determine the uniform pattern intended for scaffolding eth_chainId, then verify both middleware functions handle chainId consistently and predictably.

Written by the indexing model from the issue text.