feat(gemma4): support Gemma4 in Gemini

[douglas-reid]

The main intent of this PR is to clarify Gemma 3 classes and add Gemma 4 sample now that Gemma 4 is released. As part of that work, this PR adds gemma-4-* to the allowlist for Gemini models for use in ADK.

This PR:

• Adds gemma-4-* to the allowlist for Gemini models
• Updates docstrings for Gemma, Gemma3Ollama, and GemmaFunctionCallingMixin to clarify they are Gemma 3-only
• Adds Gemma 4 usage guidance pointing to Gemini/LiteLlm classes
• Adds hello_world_gemma4 sample using standard Gemini class
• Adds header comments and READMEs to existing Gemma 3 samples
• Adds registry non-collision test for Gemma 4 model strings
• Updates registration comments in models/init.py

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• [ X ] I have added or updated unit tests for my change.
• [ X ] All unit tests pass locally.

Please include a summary of passed pytest results.

❯ pytest tests/unittests/models/test_google_llm.py
...
tests/unittests/models/test_google_llm.py ...............................................                                                                                              [100%]
...
=============================================================================== 47 passed, 3 warnings in 4.40s ===============================================================================

❯ pytest tests/unittests/models/test_gemma_llm.py
....
tests/unittests/models/test_gemma_llm.py ......................                                                                                                                        [100%]

===================================================================================== 22 passed in 1.05s =====================================================================================

Manual End-to-End (E2E) Tests:

Ran adk run contributing/samples/hello_world_gemma4 with success

Checklist

• [ X ] I have read the CONTRIBUTING.md document.
• [ X ] I have performed a self-review of my own code.
• [ X ] I have commented my code, particularly in hard-to-understand areas.
• [ X ] I have added tests that prove my fix is effective or that my feature works.
• [ X ] New and existing unit tests pass locally with my changes.
• [ X ] I have manually tested my changes end-to-end.
• [ X ] Any dependent changes have been merged and published in downstream modules.

Additional context

[rohityan]

Hi @douglas-reid , Thank you for your contribution through this pull request! This PR has merge conflicts that require changes from your end. Could you please rebase your branch with the latest main branch to address these? Once this is complete, please let us know so we can proceed with the review.

[douglas-reid]

Hi @douglas-reid , Thank you for your contribution through this pull request! This PR has merge conflicts that require changes from your end. Could you please rebase your branch with the latest main branch to address these? Once this is complete, please let us know so we can proceed with the review.

Should be done now. Please let me know if more is needed.