fix: set appProtocol on agent Services for A2A discovery

The kagent controller creates a Service for each Agent CR but does not set
appProtocol on the port. Without appProtocol: kgateway.dev/a2a, AgentGateway's
A2A plugin cannot discover agent Services, forcing traffic through the
controller's passthrough proxy which drops HTTP headers (including traceparent).

Set appProtocol: kgateway.dev/a2a on the agent Service port so AgentGateway
can route directly to agent pods, preserving all headers and enabling
end-to-end distributed tracing.

Closes #1295

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: syn-zhu

go/internal/controller/translator/agent/adk_api_translator.go

@@ -532,9 +532,10 @@ func (a *adkApiTranslator) buildManifest(
 		Spec: corev1.ServiceSpec{
 			Selector: selectorLabels,
 			Ports: []corev1.ServicePort{{
-				Name:       "http",
-				Port:       dep.Port,
-				TargetPort: intstr.FromInt(int(dep.Port)),
+				Name:        "http",
+				Port:        dep.Port,
+				TargetPort:  intstr.FromInt(int(dep.Port)),
+				AppProtocol: ptr.To("kgateway.dev/a2a"),
 			}},
 			Type: corev1.ServiceTypeClusterIP,
 		},