.Net: Introduce Gemini Thinking Budget Configuration.

[shethaadit]

Description

PR Title:

feat(connectors-google): Introduce Gemini Thinking Budget Configuration

Closes 11645

Description:

This pull request introduces the GeminiThinkingConfig class and the ThinkingConfig property within GeminiPromptExecutionSettings to support the thinking budget feature available in Gemini 2.5 and similar experimental models.

Key changes include:

• Added the GeminiThinkingConfig class with a ThinkingBudget property for controlling the number of thinking tokens.
• Implemented validation and clamping logic for the ThinkingBudget property within GeminiThinkingConfig, ensuring it adheres to the API requirements (range 0-24576, with 1-1023 clamped to 1024).
• Added the ThinkingConfig property to GeminiPromptExecutionSettings, allowing users to configure the thinking budget for Gemini 2.5 models.
• Implemented a check within the ThinkingConfig property setter to ensure it's only used with Gemini 2.5 model IDs, throwing an InvalidOperationException otherwise.
• Included comprehensive unit tests to verify the functionality of both ThinkingConfig and ThinkingBudget, covering valid and invalid input scenarios, as well as the model ID restriction.

This enhancement enables developers to leverage the thinking budget feature for more complex tasks with supported Gemini models.

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 😄

Fixes #11645

[shethaadit]

Hi @RogerBarreto / @markwallace-microsoft, appreciate your help in reviewing this PR.