fix(router): Handle repeated headers in response

When multiple headers with the same name were sent in the response from
a subgraph, only one would be included in the response to the client
when forwarding headers. Fix it so that they're all forwarded.

router/core/header_rule_engine.go

@@ -309,9 +309,11 @@ func (h *HeaderPropagation) applyResponseRule(propagation *responseHeaderPropaga
 			return
 		}
 
-		value := res.Header.Get(rule.Named)
-		if value != "" {
-			h.applyResponseRuleKeyValue(res, propagation, rule, rule.Named, value)
+		values := res.Header.Values(rule.Named)
+		if len(values) > 0 {
+			for _, value := range values {
+				h.applyResponseRuleKeyValue(res, propagation, rule, rule.Named, value)
+			}
 		} else if rule.Default != "" {
 			h.applyResponseRuleKeyValue(res, propagation, rule, rule.Named, rule.Default)
 		}
@@ -324,7 +326,11 @@ func (h *HeaderPropagation) applyResponseRule(propagation *responseHeaderPropaga
 					if slices.Contains(ignoredHeaders, name) {
 						continue
 					}
-					h.applyResponseRuleKeyValue(res, propagation, rule, name, res.Header.Get(name))
+
+					values := res.Header.Values(name)
+					for _, value := range values {
+						h.applyResponseRuleKeyValue(res, propagation, rule, name, value)
+					}
 				}
 			}
 		}