[core] impl `mcp sampling ai sdk provider`, that sounds promising

[yaonyan]

Benefit: reuse the agent abilities of the AI SDK.

Refs

• https://ai-sdk.dev/providers/ai-sdk-providers
• https://ai-sdk.dev/providers/community-providers/custom-providers
• https://github.com/mcpc-tech/mcpc/blob/main/packages/core/src/workflow/base-sampling-executor.ts
• https://modelcontextprotocol.io/specification/2025-06-18/client/sampling