Update Gemini LLM and Embeddings to use google-genai SDK

[philschmid]

Update Gemini LLM and Embeddings to use google-genai SDK

This commit migrates the Gemini LLM and embedding integrations from the previous google.generativeai SDK to the new google.genai SDK.

Key changes include:

• Updated mem0/llms/gemini.py and mem0/embeddings/gemini.py to use the new SDK, including changes to client initialization, API call methods, and parameter handling.
• Modified tests/llms/test_gemini_llm.py and tests/embeddings/test_gemini.py to align with the SDK changes, updating mocks and assertions.
• Updated pyproject.toml to include google-genai as a dependency.

Targeted tests for the Gemini integration are passing.

Description

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Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g. code style improvements, linting)
• Documentation update

How Has This Been Tested?

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• Unit Test
• Test Script (please provide)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Made sure Checks passed

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