fix(core): map xAI native options

[rekram1-node]

Summary

• explicitly map supported @ai-sdk/xai settings to native xAI provider options
• validate reasoning effort, storage, prompt cache key, and include values
• omit invalid and unsupported xAI settings instead of forwarding arbitrary keys
• add focused mapping tests for merged catalog and user setting inputs

Testing

• bun typecheck in packages/core
• bun test test/aisdk-native.test.ts test/model-resolver.test.ts in packages/core (28 pass)