[WIP] Fix chainId conflict with Base mainnet

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

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<issue_title>The chainId: 1 (Ethereum mainnet) conflicts with the forkUrl: "https://mainnet.base.org" (Base mainnet). Base mainnet has chainId 8453, not 1. This mismatch will cause the fork to fail.</issue_title>
<issue_description>The chainId: 1 (Ethereum mainnet) conflicts with the forkUrl: "https://mainnet.base.org" (Base mainnet). Base mainnet has chainId 8453, not 1. This mismatch will cause the fork to fail.

    chainId: 8453,

Originally posted by @copilot-pull-request-reviewer in joe10832/onchaintestkit#1 (comment)</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #314

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[joe10832]

@copilot <issue_title>The chainId: 1 (Ethereum mainnet) conflicts with the forkUrl: "https://mainnet.base.org" (Base mainnet). Base mainnet has chainId 8453, not 1. This mismatch will cause the fork to fail.</issue_title>
<issue_description>@copilot chainId: 1 (Ethereum mainnet) conflicts with the forkUrl: "https://mainnet.base.org" (Base mainnet). Base mainnet has chainId 8453, not 1. This mismatch will cause the fork to fail.

chainId: 8453,

[joe10832]

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