.Net Agents - Protect against null-reference for Assistant file-search result when streaming

[crickman]

Motivation and Context

Fixes: https://github.com/microsoft/semantic-kernel/issues/9723

Must distinguish between file-search result and function-call result for assistant streaming without triggering NullReferenceException

Description

A file-search result provide any streaming result; rather, it results in an annotation on the assistant message. Still it must be distinguished from a function-call result.

Since RunStepDetailsUpdate.FunctionName property throws a NullReferenceException when being evaluated, using RunStepDetailsUpdate.FunctionResult property.

Merged fix to Open AI SDK: https://github.com/openai/openai-dotnet/pull/293

Contribution Checklist

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