fix: align Arc Testnet chain metadata

[dumanoglu1]

Summary

• update the hand-rolled Arc Testnet chain metadata to use 18 native USDC decimals
• replace the stale explorer URL with the public ArcScan testnet explorer
• add a Vitest regression check for the Arc Testnet native currency and explorer metadata

Fixes #44.

Testing

• npm ci --legacy-peer-deps
• npm run lint -- lib/circle/gateway-sdk.ts tests/arc-testnet-chain.test.ts
• npx -y -p node@22 node node_modules/vitest/vitest.mjs run tests/arc-testnet-chain.test.ts

Notes:

• Plain npm ci currently fails on the repo's existing peer dependency conflict, so I used --legacy-peer-deps.
• The local default Node is v21.6.1; Vitest's current dependency chain needs node:util.styleText, so I ran the focused test with Node v22.23.2 via npx -p node@22.