- Replace TokioConnectionProvider with TokioRuntimeProvider
- Replace ResolveError with net::NetError
- Handle Result returned from ResolverBuilder::build()
- Preserve previous Ipv4thenIpv6 default since hickory 0.26 changed
  its default to Ipv6AndIpv4, which broke localhost-based tests

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

`parse_resolv_conf` is gated to non-Apple/non-Android Unix in
hickory-resolver 0.26, so the termux branch no longer compiles on macOS.
Narrow the cfg to match where the function is available.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hickory-resolver 0.26 removed `parse_resolv_conf` on Android, so the
termux-specific path can no longer be supported via hickory. Fall back
to plain `read_system_conf` on all platforms.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>