fix(samples): Use native Gemini integration instead of LiteLLM

[mattcarrollcode]

Summary

• Fix the warning about using Gemini via LiteLLM:

  $ uv run .
  A2UI/samples/agent/adk/restaurant_finder/agent.py:107: UserWarning: [GEMINI_VIA_LITELLM] gemini/gemini-2.5-flash: You are using Gemini via LiteLLM. For better performance, reliability, and access to latest features, consider using Gemini directly through ADK's native Gemini integration. Replace LiteLlm(model='gemini/gemini-2.5-flash') with Gemini(model='gemini-2.5-flash'). Set ADK_SUPPRESS_GEMINI_LITELLM_WARNINGS=true to suppress this warning.
    model=LiteLlm(model=LITELLM_MODEL),
  

• Replace LiteLlm wrapper with ADK's native Gemini class for better performance, reliability, and access to latest features
• Rename env var from LITELLM_MODEL to GEMINI_MODEL

Test plan

• Run each sample and verify no LiteLLM warning appears

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[gemini-code-assist]

Code Review

This pull request correctly replaces the LiteLlm wrapper with the native Gemini class across several sample agent files, which should improve performance and reliability. The changes are mostly straightforward replacements of imports, environment variable names, and model instantiation. I've identified a minor inconsistency in variable naming for the Gemini model across the modified files and have provided suggestions to align them with Python's PEP 8 style guide for local variables (snake_case). This will improve code consistency and maintainability. Overall, this is a solid improvement.

[gemini-code-assist]

The variable GEMINI_MODEL should be renamed to gemini_model to follow Python's snake_case convention for local variables. This improves consistency with the style used in samples/agent/adk/rizzcharts/__main__.py and adheres to PEP 8.

Suggested change

	GEMINI_MODEL = os.getenv("GEMINI_MODEL", "gemini-2.5-flash")
	agent = LlmAgent(
	model=LiteLlm(model=LITELLM_MODEL),
	model=Gemini(model=GEMINI_MODEL),
	gemini_model = os.getenv("GEMINI_MODEL", "gemini-2.5-flash")
	agent = LlmAgent(
	model=Gemini(model=gemini_model),

References

1. The repository style guide (line 9) states that code should follow relevant style guides for each language. For Python, PEP 8 is the standard, which recommends snake_case for local variables. ^(link)