[Gemini] Add Java GeminiModelHandler class

[jrmccluskey]

Adds a Java implementation of the GeminiModelHandler, using the Java GenAI SDK to allow calling Gemini models from Java pipelines without cross-language usage.

Fixes #36253

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[jrmccluskey]

@gemini-code-assist review

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Code Review

This pull request introduces a new Gemini model handler module (sdks/java/ml/inference/gemini) for remote inference in Apache Beam, alongside generic type updates to the remote inference base classes to support arbitrary input and output types. Feedback on the changes focuses on improving the robustness and flexibility of the implementation. Key recommendations include enabling default client initialization via environment variables, adding defensive null and empty-list checks in GeminiModelHandler, removing the unused getUseVertexFlexApi parameter, updating unit tests accordingly, and removing the temporary TestReflection debugging utility.

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Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request adds a new Gemini model handler to the Apache Beam Java SDK, allowing users to call Gemini models directly from Java pipelines. It also includes necessary refactoring in the remote inference module to improve type flexibility and updates the build configuration to include the new module.

Highlights

• New Gemini Model Handler: Introduced a new Java implementation for the GeminiModelHandler, enabling direct integration with Gemini models via the Java GenAI SDK.
• Remote Inference Refactoring: Updated the remote inference module to remove strict base class constraints, allowing for more flexible input and output types.
• Integration Testing: Added comprehensive integration tests for Gemini inference, covering both text and image generation capabilities.

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[jrmccluskey]

Test failures are all unrelated on master

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[jrmccluskey]

assign set of reviewers

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