Add eth_blockNumber API endpoint support

[Copilot]

Adds support for Etherscan API v2 eth_blockNumber endpoint to query current Ethereum block height.

Changes

Workflow (.github/workflows/etherscan-apiv2.yml)

• Add eth_blockNumber endpoint option to workflow inputs
• Implement case handler routing to module=proxy&action=eth_blockNumber
• Route through v2 API endpoint with chainid=1 parameter

Documentation (ENS_CONFIGURATION.md, QUICKSTART_KUSHMANMB_ETH.md)

• Add curl examples for eth_blockNumber queries
• Correct HTML entity encoding (& → &) in shell examples

Example

# Get current Ethereum block number
curl "https://api.etherscan.io/v2/api?chainid=1&module=proxy&action=eth_blockNumber&apikey=<YOUR_KEY>"

Or via GitHub Actions:

1. Select "Etherscan API Integration" workflow
2. Choose eth_blockNumber endpoint
3. Run workflow

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