Add native ETH balance query support via Etherscan API v2

[Copilot]

Implements native ETH balance queries using Etherscan's balance endpoint. Repository previously only supported ERC-20 token balance queries via addresstokenbalance.

Changes

• Scripts: query-balance.{js,py,sh} for querying ETH balance with parameters: chainid, module, address, tag
• Documentation: ETHERSCAN_BALANCE.md covering API usage, response format, and wei-to-ETH conversion
• README: Updated with balance query examples and script references

Technical Details

• Reuses existing validation/config infrastructure (etherscan-common.{js,py})
• Python uses Decimal for precise wei-to-ETH conversion
• Bash includes 30s timeout on curl to prevent hanging
• JavaScript includes precision warning for large balances

# Query ETH balance
./query-balance.sh --apikey YOUR_KEY --address 0x123... --tag latest

# Response includes both wei and ETH:
Balance (wei): 1234567890123456789
Balance (ETH): 1.234567890123456789 ETH

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.etherscan.io
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node query-balance.js --apikey TEST_KEY (dns block)
  • Triggering command: /usr/bin/python3 python3 ./query-balance.py --apikey TEST_KEY (dns block)
  • Triggering command: /usr/bin/curl curl -s REDACTED (dns block)
• docs.etherscan.io
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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