fix(dns): decode up to four answer records (#186)

* fix(dns): decode up to four answer records

A single DNS question can return multiple answer records. Use an answer
decode limit of four in Client.StartResolve and add a regression test
covering a single-question response with multiple A records.

* fix(dns): add MaxResponseAnswers to ResolveConfig

This allows callers to explicitly declare the maximum number of answer
records to retain, removing the hardcoded limit in Client.StartResolve.

xnet.StackAsync is updated to set this limit to match the length of its
lookup-result buffer. This maintains the zero-allocation design while
fixing the issue where multiple A records were ignored.

dns/client.go

@@ -24,6 +24,9 @@ type ResolveConfig struct {
 	Questions       []Question
 	Additional      []Resource
 	EnableRecursion bool
+	// MaxResponseAnswers limits how many answer records are decoded from the
+	// DNS response. If zero it defaults to the number of Questions.
+	MaxResponseAnswers uint16
 }
 
 func (sudp *Client) Protocol() uint64 { return uint64(lneto.IPProtoUDP) }
@@ -37,8 +40,12 @@ func (c *Client) StartResolve(localPort, txid uint16, cfg ResolveConfig) error {
 	if nd > math.MaxUint16 {
 		return lneto.ErrInvalidConfig
 	}
+	maxAns := cfg.MaxResponseAnswers
+	if maxAns == 0 {
+		maxAns = uint16(nd)
+	}
 	c.reset(localPort, txid, CQueryPending, cfg.EnableRecursion)
-	c.msg.LimitResourceDecoding(uint16(nd), uint16(nd), 0, 0)
+	c.msg.LimitResourceDecoding(uint16(nd), maxAns, 0, 0)
 	c.msg.AddQuestions(cfg.Questions)
 	c.msg.AddAdditionals(cfg.Additional)
 	return nil