Add faucet-request rate limiter enforcing per-address cooldown

[thlpkee20-wq]

Description

The testnet faucet must throttle per-address requests to prevent abuse. Enforce a configurable cooldown between requests and reject early requests with a structured error.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: src/test_faucet_seed.rs
• Cooldown persisted per address

Suggested execution

• Fork the repo and create a branch
• git checkout -b feat/faucet-rate-limit
• Implement changes
  • Persist last_request_ts per address
  • Reject with FaucetCooldownActive if within cooldown
  • Emit faucet_cooldown_reject event
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test --all
• Cover edge cases
  • Cooldown exactly elapsed
• Include test output and security notes

Example commit message

feat: add faucet per-address cooldown

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

[Purity-Euphemia]

Good Day Maintainer, I have experience and would review the issue, implement a clean solution, test it, and submit a pull request.