feat(core): add configured model variant fallbacks

[rekram1-node]

Summary

• generate conservative package-aware variants for newly configured models when variants is omitted
• preserve explicit variant arrays, including variants: [] as an opt-out, across layered config
• support AI SDK and native OpenAI, OpenAI-compatible, Azure, Google, Vertex, and Anthropic transports
• map native fallbacks through providerOptions and retain -opencode.variant as the generation opt-out

Testing

• bun test in packages/core (1920 pass, 16 skip)
• bun typecheck in packages/core
• focused config, variant, and model resolver tests
• Prettier check on changed files