Support provider-prefixed Gemini model IDs

[kenrogers]

Summary

• Extract Gemini model names from LiteLLM-compatible provider-prefixed IDs such as gemini/gemini-2.5-flash, vertex_ai/gemini-2.5-flash, and openrouter/google/gemini-2.5-pro:online
• Keep malformed Vertex projects/... paths from being treated as valid Gemini IDs
• Add an OpenRouter-through-LiteLLM sample showing OPENROUTER_API_KEY, api_base, and openrouter/... model usage

Why

ADK's Gemini tool checks already normalize native model IDs and Vertex/Apigee paths, but provider-prefixed LiteLLM model IDs were left as-is. That made Google Search reject routed Gemini IDs even when the underlying model name was Gemini. This updates the shared model-name utility so existing Gemini classification paths work for provider-prefixed IDs without special-casing one provider.

Refs #2709.

Tests

• uv run --extra test pytest tests/unittests/utils/test_model_name_utils.py tests/unittests/tools/test_google_search_tool.py
• uv run --extra dev pyink --check --diff --config pyproject.toml src/google/adk/utils/model_name_utils.py tests/unittests/utils/test_model_name_utils.py tests/unittests/tools/test_google_search_tool.py contributing/samples/hello_world_openrouter/agent.py contributing/samples/hello_world_openrouter/__init__.py
• uv run --extra test python -m py_compile contributing/samples/hello_world_openrouter/agent.py

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[adk-bot]

Response from ADK Triaging Agent

Hello @kenrogers, thank you for creating this PR!

Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). You can find more information at https://cla.developers.google.com/.

Thanks!

[rohityan]

Hi @kenrogers , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh