.Net: Add responseMimeType Parameter to GeminiPromptExecutionSettings for Enhanced Output Control.

[shethaadit]

Motivation and Context

Why is this change required?
The absence of the responseMimeType property in .NET prevented users from leveraging structured output formats available in the Gemini API, causing unintended behavior in scenarios requiring JSON or controlled output.

What problem does it solve?
Adds flexibility for developers by enabling control over MIME type responses, particularly for JSON data parsing and classification tasks.

What scenario does it contribute to?

• Requests with structured response needs, such as JSON objects or controlled classification outputs.
• Seamless migration and parity with the Python SDK's responseMimeType support.

Fixes #9863

Description

This PR adds support for the optional responseMimeType parameter to the GeminiPromptExecutionSettings class, allowing for better control over the output response format in Gemini API calls. This change addresses the missing ability to specify MIME types for valid JSON, plain text, or enumerated outputs.

Contribution Checklist

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