fix(dns): validate custom resolver URLs

[ryanfowler]

Summary

• parse custom DNS server values once and reuse the validated representation
• reject invalid URL components and zero resolver ports
• support mixed-case resolver schemes and document the accepted URL forms

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins
• cargo test --locked --all-features --test cli --test formatting --test grpc --test har --test http --test install --test network --test terminal --test update --test websocket -- --test-threads=2