feat(google): add Gemini 3.8 Flash

[mikehostetler]

Summary

• Pull Gemini 3.8 Flash from the live Gemini API.
• Add verified Google metadata for the stable model ID, limits, modalities, reasoning, capabilities, prices, cutoff date, and source URLs.
• Regenerate the snapshot and add a focused metadata test.
• Keep gemini-flash-latest-high-res-exp as a separate raw API entry. No alias relation is asserted.

Evidence

• Direct Google ModelService list and get requests returned gemini-3.8-flash on 2026-09-02.
• A direct countTokens request succeeded.
• Google published the model page, launch post, pricing page, and model card on 2026-09-02.

Validation

• mix llm_db.build --check --install
• mix test: 922 passed (41 doctests and 881 tests)
• mix quality: Credo found no issues; Dialyzer found no errors

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