Introduce unified Rust domain registrar abstraction with provider fallback semantics

[Copilot]

This PR establishes a unified domain registrar library so callers can interact with multiple registrar providers through one API. It standardizes availability checks, registration flow, and failure handling across providers.

• Core API

  • Added DomainRegistrar trait for provider implementations:
    • name()
    • is_domain_available(domain)
    • register_domain(domain, years)
  • Added UnifiedRegistrar as orchestration layer over Vec<Box<dyn DomainRegistrar>>.

• Unified routing behavior

  • is_domain_available returns true if any provider reports availability.
  • register_domain attempts registration against available providers in order and falls back on provider failure.
  • Returns the selected provider name on successful registration.

• Error model and guardrails

  • Added explicit domain registration errors:
    • NoRegistrarsConfigured
    • InvalidRegistrationPeriod(u8) (enforces 1..=10)
    • DomainUnavailable
    • ProviderFailure(String)
  • Improved provider-failure propagation to retain useful context across multi-provider attempts.

• Library docs

  • Updated README to describe the crate as a unified domain registrar library and clarify its role.

let registrar = UnifiedRegistrar::new(vec![
    Box::new(ProviderA::new()),
    Box::new(ProviderB::new()),
]);

if registrar.is_domain_available("example.com")? {
    let provider = registrar.register_domain("example.com", 1)?;
    println!("registered via {}", provider);
}

Original prompt

Unified domain registrar library