Add IncludeThoughts parameter to Google Connector for accessing Gemini reasoning content

[roldengarm]

Motivation and Context

This implements the IncludeThoughts parameter for the Google connector.
See related issue here.

Description

Adds IncludeThoughts parameter to GeminiThinkingConfig to enable access to Gemini's reasoning process. When enabled, thinking content is exposed via the Items collection as ReasoningContent objects, allowing developers to access both the model's internal reasoning and final response separately.

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 😄