fix(models): register gemma 4 model names

[pandego]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Gemma 4 not registered in LLMRegistry — Gemma.supported_models() only matches gemma-3.* #5156
• Related: N/A

2. Or, if no issue exists, describe the change:

Problem:
LLMRegistry only matched gemma-3.*, so string model names like gemma-4-31b-it were rejected even though the underlying Gemini-backed Gemma path already supports them.

Solution:
Broaden Gemma.supported_models() to match the full gemma-* family and add a registry regression test covering Gemma 3 plus Gemma 4 model names.

Testing Plan

Unit Tests:

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

Passed locally:

• uv sync --extra test
• uv run pytest tests/unittests/models/test_models.py tests/unittests/models/test_gemma_llm.py

Manual End-to-End (E2E) Tests:

Not run for this focused registry fix.

Checklist

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

Additional context

This keeps the fix narrowly scoped to registry matching. It does not change Gemma request handling itself.