Fix custom chain icon fallback

[andystorrs]

Summary

• Adds the GridMind hosted logo URL to the GridMind Testnet registry entry.
• Centralizes ChainList icon selection in getChainIcon().
• Keeps existing chainSlug icons for listed chains and falls back to an HTTP chain.icon before /unknown-logo.png for custom chains.

Validation

• node -e "import('./utils/index.js').then(({getChainIcon}) => { ... })"
• node --input-type=module -e "import { pathToFileURL } from 'node:url'; await import(pathToFileURL(process.cwd() + '/constants/additionalChainRegistry/chainid-20260532.js'))"
• Generated out/rpcs.json with node generate-json.js and confirmed GridMind Testnet includes icon: https://www.grid-mind.com/logo.png.

Note: pnpm test has a Windows-local path issue in this checkout because the test harness imports raw C:\... paths as ESM specifiers; the equivalent file URL syntax import passes.