Issue: Multiple AgentgatewayPolicies targeting same backend - relevant policies only

issue1-config-dump-relevant.json

{
  "policies": [
    {
      "key": "backend/enterprise-agentgateway/aliases-policy:ai:enterprise-agentgateway/openai",
      "name": {
        "kind": "AgentgatewayPolicy",
        "name": "aliases-policy",
        "namespace": "enterprise-agentgateway"
      },
      "target": {
        "backend": {
          "backend": {
            "name": "openai",
            "namespace": "enterprise-agentgateway"
          }
        }
      },
      "policy": {
        "backend": {
          "ai": {
            "defaults": {
              "max_tokens": 500,
              "temperature": 0.7
            },
            "overrides": {},
            "modelAliases": {
              "fast": "gpt-4.1-nano"
            }
          }
        }
      }
    },
    {
      "key": "backend/enterprise-agentgateway/prompt-policy:ai:enterprise-agentgateway/openai",
      "name": {
        "kind": "AgentgatewayPolicy",
        "name": "prompt-policy",
        "namespace": "enterprise-agentgateway"
      },
      "target": {
        "backend": {
          "backend": {
            "name": "openai",
            "namespace": "enterprise-agentgateway"
          }
        }
      },
      "policy": {
        "backend": {
          "ai": {
            "defaults": {},
            "overrides": {},
            "prompts": {
              "append": [
                {
                  "role": "system",
                  "content": "APPEND_MARKER"
                }
              ],
              "prepend": [
                {
                  "role": "system",
                  "content": "PREPEND_MARKER"
                }
              ]
            }
          }
        }
      }
    }
  ]
}