.Net: Fix #13262: GeminiRequest to handle single turn requests correctly

[icyca]

Updated the CreateGeminiRequest to ensure that single turn requests end with a "user" role or no role

Motivation and Context

1. The fix is required since Gemini API rejects single turn requests ending with "model" role, but orchestration passes agent responses directly to next agents.
2. Fixes orchestration workflows with Gemini models that fail after the first agent with HTTP 400 error.
3. It contributes to multi-agent orchestration workflows using Gemini models.
4. Fixes: https://github.com/microsoft/semantic-kernel/issues/13262

Description

Removes role from single assistant messages in Gemini requests (Gemini treats no role as user input). Only affects Gemini connector, preserves multi-turn conversations, no breaking changes. Basically this is a specific case solution to keep Gemini API satisfied.

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:

[markwallace-microsoft]

@icyca this PR is still a draft, are you still working on it?

[icyca]

@markwallace-microsoft No, it is done. I just had it as draft in case it needed further discussion. I will publish it now

[icyca]

@markwallace-microsoft added unit tests