.NET Bug: OpenAIResponseAgent does not expose function-calling content

**Describe the bug**
Integrationt test does not confirm to expected result.

**To Reproduce**
Run integration tests:
`OpenAIResponseAgentInvokeTests.InvokeWithPluginNotifiesForAllMessagesAsync`

> Good: `OpenAIResponseAgentInvokeStreamingTests.InvokeStreamingWithPluginNotifiesForAllMessagesAsync`

**Platform**
 - Language: C#
 - Source: `main`
 - OS: Windows
