feat: support custom Etherscan-compatible API URLs

[kuzdogan]

Summary

Adds an optional url field to the etherscanApi chain configuration, enabling chains with their own Etherscan-compatible block explorers to use a custom base URL instead of the hardcoded https://api.etherscan.io/v2/api.

Kept separate from #2709 (remote chain config refactor) as that PR is already very large.

Changes

• SourcifyChainTypes.ts and SourcifyChain.ts (packages/lib-sourcify): Add url?: string to etherscanApi
• etherscan-util.ts (packages/lib-sourcify): fetchFromEtherscan accepts optional customBaseUrl. When provided, uses ${customBaseUrl}/api?module=contract&action=getsourcecode&... instead of the default Etherscan v2 URL
• etherscan-util.ts (server): Passes sourcifyChain.etherscanApi?.url through to lib-sourcify
• contract-creation-util.ts (server): getEtherscanApiContractCreatorFetcher accepts optional customBaseUrl. When provided, uses ${customBaseUrl}/api?module=contract&action=getcontractcreation&... instead of the hardcoded ETHERSCAN_API constant

Example usage (BattleChain Testnet)

Chain definition (extra-chains.json):

{
  "name": "BattleChain Testnet",
  "chain": "ETH",
  "rpc": ["https://testnet.battlechain.com"],
  "faucets": [],
  "nativeCurrency": { "name": "Ether", "symbol": "ETH", "decimals": 18 },
  "infoURL": "https://battlechain.com",
  "shortName": "battlechain-testnet",
  "chainId": 627,
  "networkId": 627,
  "slip44": 1,
  "explorers": [
    { "name": "BattleChain Explorer", "url": "https://explorer.testnet.battlechain.com", "standard": "EIP3091" }
  ]
}

Sourcify extension (sourcify-chains-default.json):

"627": {
  "sourcifyName": "BattleChain Testnet",
  "supported": true,
  "etherscanApi": {
    "supported": true,
    "url": "https://block-explorer-api.testnet.battlechain.com"
  },
  "fetchContractCreationTxUsing": {
    "etherscanApi": true
  },
  "rpc": ["https://testnet.battlechain.com"]
}

With url set, both the Etherscan source import (getsourcecode) and the contract creation tx lookup (getcontractcreation) will hit https://block-explorer-api.testnet.battlechain.com/api?... instead of api.etherscan.io.

Test plan

• npx lerna run build passes
• packages/lib-sourcify tests pass (158 passing)
• services/server unit tests pass (69 passing)

🤖 Generated with Claude Code

[marcocastignoli]

lgtm. we can close this since it was already merged by mistake in #2732