.Net: Add optional role parameter to 'ResponseContentPart' conversion methods

[HillPhelmuth]

Motivation and Context

Fix #13154

Description

Introduce an optional AuthorRole? role parameter to the ToResponseContentPart methods in KernelContentExtensions . Update the ToResponseItem method in ChatContentMessageExtensions to utilize this role for determining the correct content part type (Input or Output). Modify tests in ChatContentMessageExtensionsTests to ensure accurate validation of content part types based on the author role.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[HillPhelmuth]

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