feat: migrate gemini to v2

[priyansh4320]

Why are these changes needed?

This PR implements Gemini v2 client supporting rich UnifiedResponses , via ModelClientV2 protocol , should also resolve addtionalProperties bug in related issue

Related issue number

#2348

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/documentation/ to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

❌ Patch coverage is 76.56250% with 15 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/llm_clients/gemini_v2.py	82.45%	9 Missing and 1 partial ⚠️
autogen/oai/client.py	0.00%	4 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
autogen/llm_clients/__init__.py	100.00% <100.00%> (ø)
autogen/llm_config/types.py	100.00% <100.00%> (ø)
autogen/oai/client.py	53.72% <0.00%> (-4.04%)	⬇️
autogen/llm_clients/gemini_v2.py	82.45% <82.45%> (ø)

... and 19 files with indirect coverage changes

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