Allow google search tool to set different model

[lwangverizon]

** Please make sure you read the contribution guide and file the issues in the right place. **
Contribution guide.

Is your feature request related to a problem? Please describe.
Currently, the google_search tool uses the same LLM model set from parent agent for processing and summarizing search results. This creates limitations when users want to choose a different model than agent to summarize search results.

Describe the solution you'd like
Allow the google_search tool to accept an optional LLM model parameter, enabling users to specify which model should be used for processing search results. When not specified, it should default to the agent's model

Describe alternatives you've considered
Always inheriting the agent's model (less flexible for users who want different models for search)

Additional context
This enhancement improves ADK's model-agnostic design principle and gives developers more control over which LLM processes their search results, which can be important for cost optimization, performance tuning, or compliance requirements.