feat(core): route providers through native AI

[rekram1-node]

Summary

• route models.dev packages for Google, OpenRouter, and xAI through their native @opencode-ai/ai provider entrypoints
• preserve legacy flat catalog settings by projecting them to the native gemini, openrouter, and openai provider option namespaces
• retain the AI SDK fallback for all other packages
• verify both package selection and real native route construction

Testing

• bun test test/model-resolver.test.ts from packages/core (26 pass)
• git diff --check

Environment limitations

• provider plugin tests cannot start because web-tree-sitter/tree-sitter.wasm is missing
• bun typecheck is blocked by existing missing workspace modules including @ff-labs/fff-node, @opencode-ai/sdk/v2/types, web-tree-sitter, and @standard-schema/spec