AgentgatewayPolicy: Add backend tests

[npolshakova]

The new backend field on the AgentgatewayPolicy introduced in #12723 supports configuring:

• tcp
• tls
• http
• auth
• mcp
• ai

Tasks:

• Move ai backend policy tests to a new backend directory under https://github.com/kgateway-dev/kgateway/tree/main/internal/kgateway/agentgatewaysyncer/testdata/inputs
• Add unit tests for missing subfields on the backend policy under https://github.com/kgateway-dev/kgateway/tree/main/internal/kgateway/agentgatewaysyncer/testdata/inputs
• Add e2e tests for AgentgatewayPolicy backend fields covering similar cases to the envoy dataplane BackendConfigPolicy tests (https://github.com/kgateway-dev/kgateway/tree/main/test/e2e/features/backendconfigpolicy)