support tool_choice in Gemini adapter for non-live API

[omChauhanDev]

Please describe the changes in your PR. If it is addressing an issue, please reference that as well.

Fixes #3994

Change :

Added tool_choice support to GeminiLLMAdapter, converting LLMContext.tool_choice to Gemini's ToolConfig/FunctionCallingConfig format for GoogleLLMService and GoogleVertexLLMService

Note : Google's Live API doesn't support tool_config (upstream issue).

[codecov]

Codecov Report

❌ Patch coverage is 40.90909% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pipecat/adapters/services/gemini_adapter.py	31.25%	11 Missing ⚠️
src/pipecat/services/google/llm.py	66.66%	2 Missing ⚠️

Files with missing lines	Coverage Δ
src/pipecat/services/google/llm.py	45.58% <66.66%> (+0.52%)	⬆️
src/pipecat/adapters/services/gemini_adapter.py	48.36% <31.25%> (-1.26%)	⬇️

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