fix(ai): preserve Gemini agent loop parity

[opencode-agent]

Summary

• replay unsigned Gemini 3 function calls with Google's documented validator-bypass sentinel so persisted histories with missing thought signatures can continue instead of failing with HTTP 400
• forward seed, frequencyPenalty, and presencePenalty through the native Gemini generation config
• preserve nested empty-object tool schemas and their descriptions while retaining the existing root empty-schema behavior
• preserve multi-type and nullable JSON Schema type arrays by projecting them to Gemini-compatible anyOf and nullable fields instead of silently selecting the first type
• add focused regression coverage for each behavior

Validation

• bun test test/provider/gemini.test.ts (26 passed)
• bun typecheck
• bun run build
• full packages/ai suite: 479 passed, 28 skipped, 4 unrelated existing failures in Azure URL expectations and stale OpenRouter recorded fixtures

Requested by: @rekram1-node (Aiden Cline via Slack)