AgentgatewayPolicy: Add Traffic policy tests

[npolshakova]

The new traffic field on the AgentgatewayPolicy introduced in #12723 supports configuring some additional fields as a policy instead of inline filters:

• timeout
• retries
• cors
• header manipulation

Tasks:

• Add unit tests for missing subfields on the traffic policy under https://github.com/kgateway-dev/kgateway/tree/main/internal/kgateway/agentgatewaysyncer/testdata/inputs

[adity1raut]

/asssign