Run custom DNS A and AAAA lookups in parallel

[ryanfowler]

Summary

• Run UDP A and AAAA lookups concurrently with tokio::join! while keeping the existing “return any successful records” behavior.
• Reuse one reqwest::Client for both DoH queries so the two families start together without rebuilding the client twice.
• Add timing-focused unit tests for delayed A/AAAA responses in both UDP and DoH paths, plus an integration timeout check.
• Preserve the existing request-timeout error shape when DNS timeouts race the outer request budget.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features