.Net Agents - Harden Assistant handling of FunctionResultContent

[crickman]

Performing additional testing and adding integration test

Motivation and Context

Normally a ChatMessageContent created in response to a function call result has two entries in its Items collection:

1. The core FunctionResultContent item
2. A TextContent item that present the textual representation of the FunctionResultContent.Result

When only the a single FunctionResultContent is present then this results an a thread message being created without any content:

Microsoft.Azure.Migrate.Export.Copilot.AmeCopilotRequestHandler[0]
      One or more errors occurred. (HTTP 400 (invalid_request_error: empty_array)
      Parameter: content 
     Invalid 'content': empty array. Expected an array with minimum length 1, but got an empty array instead.)

Description

Handle case when a message contains only FunctionResultContent and covert result to string.

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 😄