feat: add Arc Testnet support with RPC and native currency details

[mj-kiwi]

📝 Description

Registers Arc Testnet in @metamask/delegation-deployments ahead of the delegation-framework contracts being deployed there, so the mapping and validation script are ready to go once deployment happens.

🔄 What Changed?

• Added CHAIN_ID.arcTestnet (0x4cef52 / 5042002) in src/index.ts
• Mapped arcTestnet to DEPLOYMENTS_1_3_0 under the 1.3.0 testnets, with a comment noting deployment is still pending
• Added a custom arcTestnetChain definition (RPC https://rpc.testnet.arc.network, USDC as native gas currency) to script/validate-contract-deployments.ts, since Arc isn't in viem/chains

🚀 Why?

Circle's Arc network is currently testnet-only (chain ID 5042002). Wiring up the chain config now means validate-latest-contracts can immediately confirm the deployment once the delegation-framework contracts land on Arc, without a follow-up config PR.

🧪 How to Test?

• Manual testing steps:

1. Once contracts are deployed to Arc Testnet, run:

   yarn validate-latest-contracts 5042002

2. Confirm it reports Arc Testnet succeeded

• Automated tests added/updated
• All existing tests pass

⚠️ Breaking Changes

• No breaking changes

📋 Checklist

• Code follows the project's coding standards
• Self-review completed
• Documentation updated (if needed)
• Tests added/updated
• Changelog updated (if needed)
• All CI checks pass

🔗 Related Issues

Closes #
Related to #

📚 Additional Notes

The 1.3.0 addresses are not yet deployed on Arc Testnet (confirmed via eth_getCode against DelegationManager's canonical address — currently empty). validate-latest-contracts 5042002 will fail until deployment happens; that's expected for this PR.