fix(core): derive models.dev reasoning variants

[rekram1-node]

Summary

• add providerOptions to V2 provider/model request and variant surfaces
• map models.dev reasoning_options into providerOptions-backed variants
• preserve experimental modes as separate projected models

Validation

• bun test test/plugin/models-dev.test.ts (packages/core)

Note: typecheck was intentionally not rerun after the final conflict resolution.