.Net: Fix: include taskType in Google AI embedding request (fixes #13250)

[PrathamAditya]

Motivation and Context

This PR fixes issue #13250
by ensuring that the Google AI embedding connector correctly includes the taskType field in outgoing requests when EmbeddingGenerationOptions contains a task_type value.
Previously, the connector ignored this field, causing task-specific embeddings (RETRIEVAL_DOCUMENT, RETRIEVAL_QUERY, etc.) to default to generic embeddings.

Description

Changes
Extended GoogleAIEmbeddingRequest.FromData() to include an optional taskType parameter.
Updated GoogleAIEmbeddingClient to extract task_type from EmbeddingGenerationOptions.AdditionalProperties.
Added a new overload in GoogleAITextEmbeddingGenerationService to pass EmbeddingGenerationOptions while maintaining backward compatibility.
Added unit test FromData_Should_Include_TaskType_When_Provided to verify that "taskType" appears correctly in serialized JSON.

Contribution Checklist

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

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