Add BlockVectra public RPC endpoints (9 networks, keyless)

[gjwang]

If you are adding a new RPC, please answer the following questions.

Link the service provider's website (the company/protocol/individual providing the RPC):

https://blockvectra.com

Provide a link to your privacy policy:

https://blockvectra.com/en/privacy/

Description:

BlockVectra is a multi-chain JSON-RPC and blockchain data API service.

This PR adds BlockVectra's keyless public endpoints (no API key, no sign-up) for 9 networks:

• Ethereum Mainnet (1): https://api.blockvectra.com/v1/eth_mainnet/public
• BNB Smart Chain (56): https://api.blockvectra.com/v1/bsc_mainnet/public
• Polygon (137): https://api.blockvectra.com/v1/polygon_mainnet/public
• Arbitrum One (42161): https://api.blockvectra.com/v1/arb_one/public
• Base (8453): https://api.blockvectra.com/v1/base_mainnet/public
• HyperEVM (999): https://api.blockvectra.com/v1/hyperevm_mainnet/public
• Robinhood Chain (4663): https://api.blockvectra.com/v1/robinhood_mainnet/public
• Robinhood Chain Testnet (46630): https://api.blockvectra.com/v1/robinhood_testnet/public
• Sepolia (11155111): https://api.blockvectra.com/v1/eth_sepolia/public

Each endpoint returns the matching eth_chainId. Limits are per client IP and published in https://api.blockvectra.com/v1/chains (public object); responses carry standard RateLimit-* headers. Tracking is marked limited because client IP addresses are processed for rate limiting and abuse prevention.

Documentation: https://docs.blockvectra.com/en/