fix(resolver): fix async context leak, gRPC error swallowing, add 48 tests

Fix async resolve goroutine using caller's cancelable context by
switching to context.WithoutCancel. Fix NewGRPCPlugin silently
returning broken resolver on connection failure. Fix "resolover"
typo in log field. Add doc comments to all exported symbols.

resolver/resolver.go

@@ -1,3 +1,5 @@
+// Package resolver implements DNS resolution via configurable nameservers
+// with caching, async lookup, and IPv4/IPv6 preference support.
 package resolver
 
 import (
@@ -15,6 +17,8 @@ import (
 	"github.com/miekg/dns"
 )
 
+// NameServer describes a DNS nameserver configuration including its address,
+// optional upstream chain, TTL, timeout, EDNS0 client subnet, and IP preference.
 type NameServer struct {
 	Addr      string
 	Chain     chain.Chainer
@@ -33,26 +37,32 @@ type options struct {
 	logger logger.Logger
 }
 
+// Option is a functional option for the resolver.
 type Option func(opts *options)
 
+// DomainOption sets the default domain suffix appended to single-label hostnames.
 func DomainOption(domain string) Option {
 	return func(opts *options) {
 		opts.domain = domain
 	}
 }
 
+// LoggerOption sets the logger for the resolver.
 func LoggerOption(logger logger.Logger) Option {
 	return func(opts *options) {
 		opts.logger = logger
 	}
 }
 
+// localResolver resolves hostnames via DNS using configured nameservers and a cache.
 type localResolver struct {
 	servers []NameServer
 	cache   *resolver_util.Cache
 	options options
 }
 
+// NewResolver creates a Resolver that queries the given nameservers in order,
+// with DNS caching and optional async background refresh.
 func NewResolver(nameservers []NameServer, opts ...Option) (resolver.Resolver, error) {
 	options := options{}
 	for _, opt := range opts {
@@ -161,7 +171,7 @@ func (r *localResolver) resolveAsync(ctx context.Context, server *NameServer, ho
 
 	if ttl <= 0 {
 		r.options.logger.Debugf("async resolve %s via %s", host, server.exchanger.String())
-		go r.resolve(ctx, server, host)
+		go r.resolve(context.WithoutCancel(ctx), server, host)
 	}
 	return
 }