.Net: Fix for OpenAIResponseAgent invoke with chat history

[jurepurgar]

Motivation and Context

When invoking OpenAIReponsesAgent with existing chat history that contains assistant messages the call fails with following error:

HTTP 400 (invalid_request_error: invalid_value)
Parameter: input[1].content[0]
Invalid value: 'input_text'. Supported values are: 'output_text' and 'refusal'.

The root cause of the issue is that all messages are eventually converted to InputTextPart but assistant messages have to be converted to OutputTextPart.

Description

Added isOutput parameter to ToResponseContentPart. Added test that fails before change and works after the change.