fix: set appProtocol on agent Services for A2A discovery

[syn-zhu]

Summary

• Set appProtocol: kgateway.dev/a2a on the Service port created for each Agent CR

Without appProtocol, AgentGateway's A2A plugin cannot discover agent Services.
This forces traffic through the kagent controller's passthrough proxy, which drops
HTTP headers (including traceparent), breaking distributed trace continuity.

With this one-line fix, AgentGateway can route directly to agent pods, preserving
all headers and enabling end-to-end tracing.

Test plan

• Deploy agent, verify kubectl get svc <agent> -o jsonpath='{.spec.ports[0].appProtocol}' returns kgateway.dev/a2a
• Verify AgentGateway A2A plugin discovers the Service
• Send request with traceparent header through gateway, verify trace continuity in backend

Closes #1295

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the Agent manifest translation so that the Kubernetes Service created for each Agent CR sets an explicit appProtocol, enabling AgentGateway’s A2A plugin to discover and route directly to agent Services (preserving HTTP headers for distributed tracing).

Changes:

• Set spec.ports[0].appProtocol: kgateway.dev/a2a on the per-Agent Service port.

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[Copilot]

Adding AppProtocol changes the serialized Service manifest; the golden test outputs under go/internal/controller/translator/agent/testdata/outputs/*.json currently expect the Service port to only have name/port/targetPort and will fail until they’re regenerated/updated to include appProtocol: "kgateway.dev/a2a" on the Service port(s).

Suggested change

	Name: "http",
	Port: dep.Port,
	TargetPort: intstr.FromInt(int(dep.Port)),
	AppProtocol: ptr.To("kgateway.dev/a2a"),
	Name: "http",
	Port: dep.Port,
	TargetPort: intstr.FromInt(int(dep.Port)),