chore: Call defineChain to ensure chain registration in CUSTOM_CHAIN_MAP

[d4mr]

This commit modifies the getChain function in the chain.ts file to call the defineChain function when a matching chain override is found. This ensures that the chain is registered in the CUSTOM_CHAIN_MAP even if a Chain type is already available.

---

PR-Codex overview

This PR updates chain.ts to ensure that the chain is registered in CUSTOM_CHAIN_MAP by calling defineChain for the specified chainId.

Detailed summary

• Calls defineChain(override) to register the chain in CUSTOM_CHAIN_MAP for the specified chainId.

✨ Ask PR-Codex anything about this PR by commenting with /codex {your question}